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Officials on Thursday are investigating what went wrong when what was meant to be a controlled detonation of fireworks by the Los Angeles Police Department ended in a major explosion that injured 17 people in a South Los Angeles neighborhood.

The blast came around 7:30 p.m. Wednesday in the 700 block of East 27th Street, where police had spent the day seizing a large cache of fireworks from a home. They came across about 40 homemade “coke can-sized” devices with powder and fuses on them, and 200 smaller similar devices. The bomb squad determined them to be “very volatile.”

“That’s why it was decided to really blow that stuff up here in place, in a controlled manner,” LAPD Lt. Raul Jovel said Thursday during a news briefing. The crew loaded what is believed to be less than ten pounds of the destructive material onto a commercial grade semi-truck with an iron chamber with a venting system.

But what came next was “completely unexpected,” the lieutenant said.

“During that controlled detonation, we had catastrophic failure of one of our trucks,” he added.

The blast shook the neighborhood, overturning a nearby vehicle, shattering glass and damaging homes as it sent a large plume of smoke up in the air. Sky5 was overhead and captured the explosion.

“Something happened in that containment vehicle that should not have happened. And we don’t know why, but we intend to find out why,” LAPD Chief Michel Moore said at a news conference following the blast.

Of the 17 people injured, ten are LAPD officers, one is an agent from the Bureau of Alcohol, Tobacco, Firearms and Explosives and the six others are civilian.

According to the Los Angeles Fire Department, the worst of the injuries were among civilians, who were a 60-year-old man, 85-year-old woman and 51-year-old woman with moderate injuries. The rest suffered minor injuries.

Several residents were evacuated from their homes, and police spent the night combing through the neighborhood to make sure there weren’t any hazards scattered in the area.

Jovel said protocols were followed during the detonation of the devices, and the team is trained and used to the practice.

“Our bomb techs deal with is quite on a regular basis — and particularly as we get close to the Fourth of July,” he said. “In fact, just a few days ago, they dealt with a similar incident where they also detonated a few these similar devices.”

The officers were called to the home around 8:40 a.m. that day after someone reported that there were fireworks being kept at the home.

They walked into the house to find about 5,000 pounds of fireworks stacked in boxes, totaling eight to 10 feet high in the back of the house and patio, according to the Police Department.

Investigators believe the explosive material was bought from out of state, and transported to South L.A. to be resold to community members, according to the police chief.

Arturo Cejas, a 27-year-old man in the home, was taken into custody, and he faces charges related to possession of a destructive device, LAPD said. He may also face additional charges because there was a 10-year-old in the home where the fireworks were being kept.

The explosion Wednesday rocked the neighborhood.

“I didn’t think it was a firework. It sounded more like an earthquake,” said one South L.A. resident, who asked only to be identified as Kenri. “I’ve never felt a firework like that, it never shakes the whole house.”

Neighbor Maria Velsaquez was told to leave her home for a couple hours before Wednesday’s detonation, but was still displaced late Thursday morning. 

“They said it was only going to be an hour and a half to two hours, and they didn’t even tell us it was going to be an explosion or anything,” Velsaquez said. “They said it was gonna sound like a thump.”

The police chief said officers knocked on nearby residents’ doors to evacuate the area surrounding the truck ahead of the detonation.

“I can’t really blame it on LAPD or anything, but they’re professionals. They should have done this in a vacant lot,” Velsaquez said.

L.A. City Councilman Curren Price had similar thoughts.

“One of the main questions that myself and my constituents want to know is why this happened in a densely populated residential area?” the councilman said in a statement

Police said the explosion will be investigated over the course of the coming days. Federal investigators are expected to arrive at the scene later Thursday.

“The Chief of Police has requested ATF to conduct a very thorough investigation to determine exactly what happened,” Jovel said.