For 19 days, a bomber held Austin, Texas, hostage - residents of the city paralyzed by the fear of unpredictable strikes.
By the time authorities caught up with the suspect, six bombs had detonated, two people were dead and five were injured.
A year has passed, but for the first time, ABC News' "Nightline" takes you inside the investigation that brought down one of the most prolific bombers in American history, through the eyes of law enforcement on the ground - the Austin police, the bomb squads, FBI and ATF agents - and by revealing never-before-seen details on how they found him.
"In the history of this country, we haven't had a serial bomber that planted this many devices in a 19-day period," said Fred Milanowski, the ATF special agent in charge of the Austin probe.
Austin, home to the South by Southwest festival and the University of Texas, is a colorful capital city known for its live music and barbecue.
But the morning of March 2, 2018, changed all that.
That morning, 39-year-old Anthony Stephan House opened a package left outside of his door and unknowingly detonated a bomb.
"My neighbor - something exploded. There's blood everywhere...it sounded like an explosion," one of House's neighbors told 911 during a call obtained by ABC News.
Police rushed to the scene. It was the first time the city's bomb squad had dealt with a live bomb.
"After we were there a few minutes, we realized...the type of injuries...that the victim had-sustained," said Austin PD bomb technician Caine Johnson. "I think at that point, I think we realized we had some sort of bombing."
House died from his injuries at a local hospital.
It was an unusual method for murder, one that brought in teams from the FBI and ATF to analyze the scene. But at the time, all agents on the House case seemed to agree that it was likely a one-off fatal bombing.
"There is no information to believe that this is anything other than an isolated incident," Austin Police Chief Brian Manley said at the time. "[We] came up with a theory on the scene."
"The Austin police department's Organized Crime Division had conducted a raid in the weeks leading up to this bombing on a house on that very same street," he continued. "The vehicles were very similar in appearance. So we did have a very early [theory that] this was an intentional act, but they got the wrong house, the wrong victim."
Ten days went by, and then on March 12, 2018, the first day of the annual SXSW festival, police got a call about a second package exploding at a residence in the northeast section of the city. Seventeen-year-old Draylen Mason was dead and his mother was critically injured.
"When I heard the bomb squad commander say, 'We have another device,' I thought, 'In the United States? This normally is what we see happening overseas,'" said FBI agent Mike Call.
Then, around 11:50 a.m. that very same day, police received calls about a third package exploding and severely wounding 75-year-old Esperanza Herrera.
"Even before looking at the components of the device...we could tell that the handwriting on that package was immediately similar to the handwriting on the package from earlier in the day," said ATF analyst Jeff Kennedy.
When they arrived at the third bombing scene, Special Agent Chris Combs, the FBI's top agent in the area, said he turned to Manley and acknowledged that they had a serial bomber on their hands and that they needed to sound the red alert. Manley went on to deliver a terrifying message to the media and the people of his city.
"It is important that people be vigilant and be aware of things that look suspicious," Manley said at the time. "If you have had a package show up in your home, and you were not expecting a delivery. If the package that is delivered to your doorstep looks suspicious in any way, call 911, report it."
Suddenly, panicked residents overwhelmed police with suspicious package calls.
On March 18, Manley addressed the public again, saying the bomber was trying to send a message with each attack.
"We hope this person or persons is watching and will reach out to us before anyone else is injured or before anyone else is killed out of this event," he said at the time.
Hours later, the bomber responded, not with a phone call but with more terror.
Police said a fourth device detonated that evening when two young men, aged 22 and 23, were walking on a sidewalk in the Travis County neighborhood. The bomb was set off by a tripwire and was placed near a fence, concealed by a red sign, police said.
"I walked over and there was a bicycle in the middle of the street...and these two young men were right here in the sidewalk," said Austin PD Senior Patrol Officer Seth Model, an Iraq war veteran, who was among the first on the scene.
"And [I] look at them, look at the fence and realized that I was looking at shrapnel, and then it just clicked. I had to start thinking like I was a soldier again, in Iraq and I realized that this was a bomb," he continued. "I just started walking slowly and I take out my flashlight because it's getting darker and darker, and I shine my flashlight just looking and I just start doing what a soldier would do and I was like, there's gotta be something here and I just start scanning the area."
ATF Special Agent and explosives enforcement officer Alex Guerrero explained that the bomber had placed the device on the ground and covered it somewhat with the sign, ran a tripwire about "three inches off the ground" and anchored it into a metal stake. The sign read "Drive Like Your Kids Live Here."
The tripwire setup changed everything for law enforcement. They said it was a technique usually seen in war zones.
"He could have put more tripwire devices out there in the area knowing that we...would be out there clearing the scene to make sure there was no secondaries," Guerrero said.
FBI agent Call said they realized they had no safe way to clear the neighborhood.
"Alex [Guerrero] and I were in the same house, standing in a house where there was shrapnel and I.E.D. components in their backyard, and telling the citizens, 'You can't go in your backyard. It's a crime scene.' And I remember clearly the citizen saying, 'We're scared. We're not even sure if we can leave our house safely,'" Call said.
The tripwire, officers said, also told them that the bomber was becoming more sophisticated and confident.
"It was a signal," said Chris Combs of the FBI. "We also were very concerned, because of the change in the bomb, the sophistication of the bomb, which now led us back to thinking, 'Could this be a terrorist?'"
By that point, 16 days since the first explosion, police said none of their suspects had panned out.
"We had people of interest throughout this investigation. And the night of the tripwire device, we knew where those people were, and they were nowhere near that," said the ATF's Milanowski. "So, unfortunately, the morning we started processing that scene, we were at square one."
On March 19, a fifth explosion was reported at a FedEx sorting facility just outside of San Antonio. One employee had been injured.
"When the FedEx package detonated at the facility, that provided critical evidence that...the subject had...changed his M.O.," said FBI agent Justin Wilson. "The bomber is diversifying...his methods."
"That elevated our concerns tremendously because now you're shipping something, and before we knew they were placed," he added. "And we didn't know how many."
Sending a package through FedEx was the bomber's third method of delivery after first placing packages at residences and then using the tripwire. Police said the package that caused the FedEx facility explosion was on its way to an address in Austin and had been sent from a "Kelly Killmore."
They also said that they realized the bomber was becoming bolder and jeopardizing the lives of many more people. Putting a bomb into the mail or shipping systems meant the device could end up on a plane and the consequences could be devastating.
Detectives said they felt they had to find the bomber fast because they were running out of time. FBI Special Agent Michael Call said he decided to call the FedEx facility in Austin.
"[I] described the [San Antonio] package to the manager at the [Austin] FedEx facility and he said, 'Yes, we have that particular box...It's on the dock for outbound delivery," Call said. "So, at that point, I told the manager, 'That is a live improvised explosive device. Evacuate the building and then call the bomb squad.'"
By dawn on the morning of March 20, 2018, the Austin PD bomb squad was on the scene of the FedEx facility in Austin.
"We immediately sent one of our robots inside," said Rob Nunez, a senior police officer with the Austin PD Bomb Squad. "With the robot, were able to use the cameras and the manipulator arm to look at the package, get the tracking numbers off of it, and see that, yes, this was it."
Police said the package that the bomb squad safely intercepted had been marked for delivery that same day. The tracking number lifted from the package led authorities to a FedEx store in Austin.
When they took a look at the store's surveillance footage, ATF agent Dan Mueller said the suspect "was wearing a disguise."
"He was wearing gloves and everything else," he said. "The interviewing agents talking to the FedEx employee said [the suspect] went and got into a red Ford Ranger."
"The fact that he walked...brazenly into a FedEx facility to drop off those two packages led to the identification of him," ATF agent Matt Abowd said. "That was his biggest mistake."
Authorities reached out to Home Depot, where they believed the bomber might have purchased his supplies. The Home Depot's Organized Retail Crime Division works in tandem with law enforcement for these situations.
Jeremy Greenleaf, Home Depot's corporate manager of investigations, said when his team received the suspicious packages, they were "able to identify those gloves, come up with the product identification number, and... the sign" reading "Drive Like Your Kids Live Here" that was used at the tripwire scene.
Then, his team was able to "generate the one receipt purchased here" in Round Rock, Texas.
After scouring hours of surveillance footage, Home Depot security located the suspect.
"He walked into the building, he asked the door greeter some questions...you could tell he was kind of asking where certain items were," said Home Depot organized retail crime investigator Terry Pruse. "The greeter directed him to that area, and minutes later, he came back with some of the items that we were looking for."
According to the Home Depot investigators, the suspect was seen on surveillance purchasing the red sign and gloves he later wore to drop off the packages at FedEx, then he exited the Home Depot and got into his vehicle - a red Ford Ranger.
As that information was being called into the command center, a team of FBI analysts immediately started searching for the person who might have both purchased those supplies and owned that red truck.
FBI Staff Operations Specialist Jordanna Nesvog launched into the painstakingly thorough process of combing through names and records to find a match.
"I was systematically going through each name, and just looking at what kind of vehicle would this person have," Nesvog said. "And I got to about the seventh name. And it was the bomber's name, and I conducted a search to find out what kind of vehicle he had registered to him if he had one. ... It was exactly the vehicle that we were looking for."
Going through a careful process of elimination, she quickly found the name of their suspect even as she was still getting information phoned in via the conference call.
"It was a heart-stopping moment," Nesvog said. "And I got goosebumps. I knew. I knew this is something good."
Finally, law enforcement had the identity of a likely suspect who had eluded them for 18 days: 23-year-old Mark Conditt. Police said he was a young man with no criminal record, who came from a family with deep ties to the community and a long-standing connection to its church.
As they pieced together the suspect's profile, his known addresses, where he might be spending his time, prosecutors and investigators were racing to get search warrants approved. By that point in the day, it was getting late.
"So we decided that we were going to hold and execute that warrant at sun up [on March 21, 2018], without making any further notice," Chief Manley said.
But they never got to sun up.
"About midnight we had decided we're going to take the house the next morning," said FBI Special Agent Justin Wilson. "It was decided then, 'Go home, get a few hours' sleep, reconvene in the morning.'
I went ahead and headed home, which, ironically, is about a mile-and-a-half, two miles away from where [the suspect] was located."
The bomber had gone dark, but then, in a stroke of luck for law enforcement, he briefly turned on his cell phone and police were able to ping his location.
"And I had just gotten home - started to lay down when my phone rang from the command post and said, 'We got him,'" Wilson said.
They tracked him to a hotel parking lot in a suburb north of Austin. Wilson was sent to the parking lot to stake him out.
"I had darked out my lights. I sat up. I could see the bomber's vehicle from an oblique angle," he said. "I'm going through in my head, 'How's this going to play out...you are dealing with a bomber.'"
Just down the road, members of the city's SWAT team were staged in another nearby parking lot. They had raced to that spot after originally being sent home to get a few hours of sleep. When they got there, they didn't even have the armored trucks they usually use on dangerous operations.
"We had to stop him that night," said Robert Justesen, a senior police officer with the Austin PD SWAT team. "He didn't care who he hurt or killed at this point."
"The longer I sat there...you just got this feeling over you that he's finalizing his thoughts," Wilson said. "I got an eerie feeling that it wasn't going to end well."
Wilson said he watched Conditt in his truck, then he watched him back out of the parking spot. He followed him, with Austin Police and SWAT directly behind him.
"He proceeded south on the service road [and] turned out to go to the Frontage Road of I-35," Wilson remembered. Then the suspect stopped at a red light.
"We had quite a few people pull up behind him," said Jeff Williams of the FBI special operations group. "If you look at the videotape later on, I mean, you see a pretty long train. At that point, the guy knew."
That's when SWAT decided it was time to act. They planned to hit his van hard enough to disable it - a standard maneuver, but they were traveling in standard vans and this was no ordinary suspect on the run.
"One of our biggest fears is on impact, it was going to explode," said Lt. Katrina Pruitt, commander of the Austin Police SWAT. "I told them, 'You cannot let him get on the interstate. Whatever you do, you've gotta stop him before he gets on the interstate.'"
Police said SWAT hit his vehicle, causing him to roll off the road, making him unable to flee. Then, the SWAT team came out of their van and approached.
All of a sudden, there was a boom.
As police had feared, the bomber set off another explosive, this time inside his vehicle.
"I saw the flash and I felt shrapnel or debris or whatever hit my face, and I stepped back," Justesen said.
Leighton Radtke, another member of the SWAT team that night, was one of the first to go up to the windshield and looked inside.
"I quickly recognized that our suspect was no longer a threat at that point," he said. "I'm really lucky that I didn't lose anybody."
Conditt was dead. After 19 days of tracking the elusive killer, the terror had finally ended.
"There was a moment of relief in that we had at least taken who we believed to be our prime suspect," Chief Manley said. "We could account for him. But we didn't know what he had been doing. Whether there were more bombs still in the community somewhere and we still had to investigate whether he was acting solo or in concert with anybody."
They had to go to the house the next morning to find out. FBI's Combs said, "That was going to be a very dangerous SWAT hit."
Darla Roessler and her family live just across the street from Conditt. Their videos show the SWAT team entering the suspect's residence.
"I went through different phases," Roessler said. "I really would look over there and it didn't really sink in. I guess that's denial. And then the negotiating with God, 'Why did he not harm us?'"
Conditt lived in a room in the back of the house, where police said they found enough weapons materials to blow up the entire block. It was where he had masterminded the bombing spree.
"He had the capability to build much bigger devices," said ATF agent Fred Milanowski. "All that stuff was in his house, along with the wig and the gloves and everything that we tied him to this investigation was in his bedroom, in that house."
Manley reflected on the public's yearning for a motive.
"That's what everybody wants to know, right? ... I'd love to know why. But we're left with what we're left with," Manley said. "The reality is there's no answer and there's no reason that could ever make any of this okay."
U.S. Attorney John Bash said law enforcement "reviewed all of the evidence they could get their hands on, all his electronic devices, everything in that house. And they have found no evidence of links to a hate group or a terrorist group, no evidence of being influenced by any recognized ideology."
"It may be a mystery [as to] what was internally motivating him forever," Bash added.
Authorities say they still don't have a motive, but the bomber did leave behind a 25-minute manifesto recorded on his phone.
In his manifesto, District Attorney Margaret Moor said he threatened to "walk into a crowd at McDonald's and blow up everyone that's in there with this last bomb that's in my pocket."
"He purposely said he has no remorse," added FBI agent Chris Combs. "On that tape, there is no mention of terrorism, hate crime. He says he's a psychopath and he likes killing people."
Law enforcement officials say they intend to keep the manifesto recording under wraps.
"What should be remembered is the victims and the first responders like the chief's SWAT teams that risked their [lives]," Combs said. "There is no value in that audiotape going out there, except being held up by other people who want to do bad things."
As for the friends and families of Conditt's victims, they are left with shockwaves of pain from loss and never knowing why.
"There's always moments where I...don't believe that this has actually happened," said Patrick Slevin, victim Draylen Mason's music teacher. "We've lost someone truly special."
They said they cling to memories and old photos.
"I still feel like I'm walking in a fog. I miss him so much," Melanie House Dixon, the mother of Conditt's first victim, Stephan House, said. "I just miss his essence. I just miss him. And that's not getting better. I thought maybe at first it was going to be okay. But as time has gone on, it's just gotten worse for me."
A year later, the police and federal agents who hunted the Austin bomber can tick off the hours of investigation they took on and the terabytes of data that were reviewed from Conditt's electronic devices, but they still have no idea why this man decided to terrorize this city and ruin so many lives in the process.
"To be perfectly transparent, we don't [know] a lot more than we knew, you know, a week or two after he killed himself," said the ATF's Milanowski. "We don't know why he picked these targets. And we probably will never know that. All investigative information leads us to believe that they were randomly selected. And so, you know, we just don't know. We never were able to make a connection...so we do know that he was a psychopath. We do know that he enjoyed killing people. We just don't know why he picked these people."
For his part, the FBI's Combs said the Austin bomber probe is a testament to good detective work, but that's not what has stayed with him.
"It's a double-edged sword," he said. "I think of when I was on those bombing scenes and those poor people that got killed and got injured. ... I agree with Fred, the sense of accomplishment, that we were able to come together in 19 days, track this bomber down and stop him from doing any more bombs. I could not be prouder of that. But it's still hard to be here and think about all those poor people that got hurt and injured."
ABC News' Halley Freger, Lauren Effron and Allie Yang contributed to this report.
This report was featured on the Tuesday, March 12, 2019, episode of ABC News' daily news podcast, "Start Here":
"Start Here" is the flagship daily news podcast from ABC News -- a straightforward look at the day's top stories in 20 minutes. Listen for free every weekday on Apple Podcasts, Google Podcasts, iHeartRadio, Spotify, Stitcher, TuneIn, or the ABC News app. On Amazon Echo, ask Alexa to "Play 'Start Here'" or add the "Start Here" skill to your Flash Briefing. Follow @StartHereABC on Twitter, Facebook and Instagram for exclusive content, show updates and more.
A city in fear: How police, federal agents took down the Austin bomber