A mass amount of destruction remains after an explosion triggered by a bomb squad detonating homemade fireworks in the South Los Angeles neighborhood, with crews pulling a 500-pound lid to the failed explosive container from “somebody’s yard” on the second day of cleanup Friday, officials said.

The hulking lid was found three to four blocks from the center of the blast on East 27th Street and towed from the scene for further analysis, said Michael Hoffman, an assistant special agent in charge with the Bureau of Alcohol, Tobacco, Firearms and Explosives.

It’s unclear how long it will take to clear the scene that extends across several blocks so that residents can return home, according to Hoffman.

Seventeen people were hurt Wednesday night — including nine Los Angeles police officers and a federal agent — in the blast, which also flipped and damaged cars and smashed windows in homes and a laundromat. The explosion was heard blocks away.

It could take days to determine why the material exploded inside a spherical containment vessel on a tractor-trailer, tearing the rig apart in what was supposed to be a safe operation to handle explosives that were too unstable to remove from the Historic South-Central neighborhood, where tons of illegal fireworks were discovered.

ATF experts from across the country arrived at the scene Thursday night and Friday morning to investigate the cause, and planned to work through the holiday weekend into early next week.

“What we are trying to do at this point, to support getting folks back into their residences, is to shrink the footprint of the actual scene,” Hoffman said at a news briefing Friday afternoon.

Investigators are gathering pieces of evidence in the streets surrounding the blast, as well as removing some vehicles. Some cars were too damaged to be driven and are being towed out, but residents can retrieve those that are in working order.

“Some of those vehicles will be released to their owners. So if there’s anyone that has been affected by their vehicles, or need access to their vehicles, please come to the to the outer perimeter at 27th and San Pedro [streets],” said LAPD Cmdr. Ruby Flores.

Officials are also escorting people into their residences to retrieve essential items like medication.

A shelter has been set up at the Fred Roberts Recreation Center, at 4700 Honduras St. in the Central-Alameda neighborhood. Officials are still working to establish an assistance center where residents can access services and recovery information, but a directory of resources has been posted online.

Authorities were still unable to provide any answers Friday as to why the blast occurred.

Experts say the explosion was highly unusual, especially for a law enforcement agency with the size and resources of the Los Angeles Police Department. The blast could have been the result of human error — such as not correctly sealing the vessel or over-loading it with material — or a defect in the equipment like a micro-fissure that has grown with time and use. Or both.

“They were very lucky that nobody got killed here,” said Glenn Corbett, a fire science professor at John Jay College of Criminal Justice in New York.

Bomb squads nationwide will likely now double-check their own equipment in remote locations, to be sure there aren’t potential defects in their containment vessels, which cost tens of thousands of dollars, that could hurt more people, Corbett said.

The LAPD did not answer questions Thursday seeking information such as how old the containment vessel was and how many times it had been used and inspected. But Chief Michel Moore previously said “clearly, protocols were followed and pursued.”

“But something happened in that containment vessel that should not have happened, and we don’t know why,” Moore said. “But we intend to find out why.”

Less than 10 pounds of material was placed in the chamber, far less than its safety rating, Moore said. He described the material as 40 home-made devices the size of Coca-Cola cans with simple fuses and 200 smaller but similar devices.

“This vessel should have been able to dispose of that material,” he said. But instead there was a “total, catastrophic failure of that containment vehicle.”

Typically, the heavy-duty hydraulic doors will latch and lock — usually by remote — and the detonation is instantaneous. The displaced blast wave is absorbed by the vessel and released outside safely.

“You wouldn’t even know,” said Sean Dennis, president and co-founder of the United States Bomb Technician Association, besides a muffled ding and a light hiss when some of the pressure is released.

The LAPD has not said what company made the containment device. Attempts to reach several manufacturers Thursday were unsuccessful.

The explosion came after police had spent the day disposing of about 3,000 to 5,000 pounds of commercial-grade fireworks that were found in the home following an early-morning tip. Police found some of them on a patio in cartons stacked 8 to 10 feet high, Moore said.

Fireworks are illegal to sell or possess in Los Angeles and in unincorporated areas of the county.

A resident, Arturo Cejas, 27, was arrested on suspicion of possessing a destructive device, but he also may face charges of child endangerment because his 10-year-old brother was in the home, Moore said.

It wasn’t immediately known whether Cejas had an attorney who could speak on his behalf.

The fireworks cartons were marked “Made in China.” The police chief said they were bought out of state in order to sell them in the neighborhood for use on the Fourth of July.

Peter Diaczuk, a forensic science professor at John Jay, said the size of the blast indicated high-explosive devices that wouldn’t be used for a local Independence Day celebration.

As the holiday approaches, fire departments statewide are confiscating tens of thousands of pounds of illegal fireworks and warning users that setting them off could lead to devastating wildfires. Los Angeles police on Thursday announced that they seized another ton of illegal fireworks from a shipping container and the bomb squad trucked it to a storage facility.

In L.A., Moore said on Wednesday that three box trucks and a 53-foot trailer were brought in to load up the fireworks at the South Los Angeles home and move them to a safe location. However, the “improvised explosives” that were also found were too dangerous to move, the chief said.

There was a 300-foot boundary around the vessel — a typical safety range — and police had knocked on doors before the blast to evacuate homes on both sides of the street where the truck was parked. But firefighters later found people in some of the homes, officials said.

It’s not unusual for bomb squads to not know exactly what comprises the homemade explosives they are detonating, Diaczuk said.

“You don’t know what you’re dealing with. You don’t know what this character made,” he said. “It doesn’t come with ingredients in it like when you go to the grocery store.”