Darion Marcus Aguilar, 19, took a taxi to the Mall in Columbia in suburban Baltimore on Saturday morning and entered the building near Zumiez, a shop that sells skateboarding gear. He went downstairs to a food court directly below the store, then returned less than an hour later, dumped the backpack in a dressing room and started shooting, police said.
Shoppers fled in a panic or barricaded themselves behind closed doors. When police arrived, they found three people dead - two store employees and Aguilar, who had killed himself, authorities said.
The shooting baffled investigators and acquaintances of Aguilar, a quiet, skinny teenager who graduated from high school less than a year ago and had no previous run-ins with law enforcement. Police spent Sunday trying to piece together his motive, but it remained elusive.
Aguilar, who had concealed the shotgun in a bag, fired six to nine times. One victim, Brianna Benlolo, a 21-year-old single mother, lived half a mile away from Aguilar in the same College Park neighborhood, but police said they were still trying to determine what, if any, relationship they had.
Although they lived close to Maryland's largest university, neither was a student there. Aguilar was accepted last February to Montgomery College, a community college in the Washington suburbs, but school spokesman Marcus Rosano said he never registered or attended.
The other employee, Tyler Johnson, did not know Aguilar and did not socialize with Benlolo outside of work, a relative said.
Tydryn Scott, 19, said she was Aguilar's lab partner in science class at James Hubert Black High School and said he hung out with other skaters. She said she was stung by the news.
"It was really hurtful, like, wow - someone that I know, someone that I've been in the presence of more than short amounts of time. I've seen this guy in action before. Never upset, never sad, just quiet, just chill," Scott told The Associated Press. "If any other emotion, he was happy, laughing."
Aguilar graduated in 2013, school officials confirmed.
The Prince George's County Police Department said it received a missing persons report for Aguilar at about 1:40 p.m. Saturday, more than two hours after the mall shooting. Officers went to Aguilar's home to speak with his mother about 5 p.m. and saw Aguilar's journal. The portion the officer read made him concerned for Aguilar's safety, the department said.
Police began tracking Aguilar's phone and soon discovered it was at the mall.
Howard County Police Chief William McMahon said the journal expressed general unhappiness, but he did not give any specifics about the writings.
"There are a lot of unanswered questions," McMahon said.
The police chief said there has been speculation about a romantic relationship between the gunman and Benlolo, but investigators have not been able to establish that.
Aguilar purchased the 12-gauge shotgun legally last month at a store in neighboring Montgomery County.
At his home, officers also recovered more ammunition, computers and documents, police said.
The home is in a middle-income neighborhood called Hollywood, near the Capital Beltway. No one answered the door Sunday morning. There was a Christmas wreath on the front door and signs that read "Beware of Dog."
Aguilar and his mother rented the home. Sirkka Singleton, who owns the property with her husband and lives a block away, said they use a property manager to find tenants and have never met the Aguilars. She declined to say who the property manager was.
A roommate who answered the door at Benlolo's home confirmed that she lived there but declined to comment further. Two police officers went into the home after he spoke briefly to a couple of reporters.
Residents described the neighborhood as a mix of owners and renters, including some University of Maryland students.
A man who answered the phone at Johnson's residence in Mount Airy, northwest of Baltimore, said the family had no comment. The victim's aunt told a local television station that she did not believe her nephew knew Aguilar.
Sydney Petty, in a statement to WBAL-TV, said she did not believe her nephew had a relationship with Benlolo.
"Tyler didn't have anything beyond a working relationship with this girl, and he would have mentioned it if he did, and we're just as confused as anybody," Petty said.
She said her nephew also worked at a drug rehabilitation center in Mount Airy, for which she served on the board.
Five other people were hurt in the attack, but only one was hit by gunfire - a woman who was hit in the foot upstairs near Zumeiz. All were released from hospitals hours later.
The mall was to reopen Monday afternoon.
Benlolo's grandfather, John Feins, said in a telephone interview from Florida that his granddaughter had a 2-year-old son and that the job at Zumiez was her first since giving birth to her son.
"She was all excited because she was the manager there," he said.
He described his daughter's family as a military family that had moved frequently and had been in Colorado before moving to Maryland about two years ago. He said his granddaughter was on good terms with her son's father, and they shared custody.
"I mean, what can you say?" he said. "You go to work and make a dollar and you got some idiot coming in and blowing people away."