A dozen firefighters were injured, including 11 who were hospitalized with burns after a "massive fireball'' engulfed them during the explosion Saturday, according to the Los Angeles Fire Department. A total of seven have been released.
The Los Angeles Fire Department's arson and counter-terrorism unit, the criminal conspiracy section of the Los Angeles Police Department's Major Crimes Division and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives are investigating whether criminal conduct was involved in the explosion.
RELATED: Downtown Los Angeles fire: Investigation continues into massive explosion that injured 12 firefighters
"The National Response Team ... along with ATF special agents from the Los Angeles Field Division, were activated today to join the investigation of the fire,'' the ATF said in a statement released Monday morning.
NRT experts arrived Monday to begin processing the scene.
The NRT was activated at the request of the Los Angeles city officials, according to the ATF.
"ATF is committed to working alongside Los Angeles Fire Department to determine the origin and cause of this fire that tragically injured firefighters,'' said Monique Villegas, special agent in charge of ATF's Los Angeles Field Division. ATF will provide whatever resources are necessary to thoroughly investigate and provide answers.''
Carbon dioxide and butane canisters were found inside the building, but it is not yet clear if they contributed to the explosion.
Investigators are looking into a fire at the same business in September 2016.
LAFD Capt. Erik Scott said "one significant explosion" shook the neighborhood around 6:30 p.m. Saturday. Firefighters inside had to run through a wall of flames he estimated as 30 feet high and wide, and those on the roof scrambled down a ladder that was engulfed in flames.
The site of the explosion, a single-story building at 327 Boyd St., houses Smoke Tokes Warehouse Distributor, a supplier for those who make butane honey oil,'' according to Scott.
Video captured firefighters on the roof scrambling down ladders with their protective coats on fire. The wall of flames shot out the building and burned seats inside a fire truck across the street.
"You see 'em crawling through a fire ball and it's gut-wrenching to watch. But they kept their wits about them, and they did what they were trained to do. You can just imagine the heat, the temperature of what they're experiencing, the amount of pain they're probably going through in that moment," Los Angeles Fire spokesperson David Ortiz said. "That's something that's gonna take its toll and it's something that you carry with you into your retirement and it stays with you the rest of your life."
There was light to moderate smoke when firefighters entered the one-story building in the city's Toy District and went on the roof - normal procedures to try to quickly knock down any flames.
LAFD Chief Ralph M. Terrazas said one of the firefighters inside the building thought things didn't seem right - the pressure from the smoke and heat coming from the rear of the building were increasing. He directed everyone to get out, and they quickly started exiting the building as it was rocked by the explosion.
"When something out of the ordinary starts to happen, they were describing a loud, rumbling dead engine sound, the hair on the back of your neck starts to stand up and you say, well if something doesn't feel right, it's time to bail out," LAFD spokesperson David Ortiz said. "Right as they were bailing out, that's when the fireball engulfed the firefighters."
Some of the 11 firefighters suffered "obvious damage and burns'' in the explosion and were taken to County-USC Medical Center, according to Scott,
who said two of them were listed in critical but stable condition. The 12th firefighter was treated and released from an emergency room Saturday for a minor extremity injury.
Dr. Marc Eckstein, medical director for the LAFD and a physician at County-USC Medical Center, said the 11 hospitalized firefighters arrived awake and alert, but two were put on ventilators due to smoke inhalation and four were sent to the intensive care unit for burns.
Most of the burns, Eckstein said, were on their upper extremities.
On Sunday, the two firefighters were removed from ventilators, but remained in intensive care, Prange said.
L.A. Mayor Eric Garcetti, at a briefing Sunday night, said: "The good news is that everybody is going to make it,'' but added, We have a lot of firefighters who are shaken up.''
Terrazas said the mayday call, which is used only when a firefighter is "down, missing or trapped,'' was "the kind of call I always dread.''
Terrazas said the injured men, who were from Engine No. 9, realized something was wrong when they were inside the building but could not escape in time to avoid the blast. Their fire engine parked outside was charred, and the aerial ladder was damaged -- with eyewitnesses saying firefighters on that ladder climbed down with their coats on fire.
Multiple ambulances and fire companies were called to the scene, with more than 230 firefighters responding and establishing a treatment area just east of the building.
The fire, which spread from the narrow one-story building where it originated to neighboring businesses, was knocked down at 8:08 p.m.
The cause of the fire "is of paramount concern,'' Scott said.
Earl King, a 64-year-old man who lives in an alley a block from the building that went up in flames, said at first the smoke was so minor he thought it was just a trash can fire.
"The smoke was getting bigger,'' he said. "And then all the sudden there was a big 'ole popping sound...POP, POP, POP...That's when, BOOM! And then we can feel it -- you know that little vibration.''
The sound reminded him of a large train chugging right toward him, he said.
"It scared the hell outta me,'' he said. "And then when we looked up we seen all the smoke, and the ashes coming down with fire on 'em.... It was no joke. It was no joke.''
King said the blaze seemed to be in a complex that includes a vape shop warehouse where he's worked as a day laborer.
"We be doin' their containers,'' he said. "You know, unload their truck.''
King said when he was working in the building he noticed plenty of flammable materials.
"A lot of those warehouses have chemicals, you know the stuff, like butane for lighters or whatever,'' he said.
Police described the area where the explosive fire occurred as "Bong Row'' because of a large number of cannabis, CBD and pipe businesses.
City News Service contributed to this report.