At least 17 people were injured as a Los Angeles bomb squad botched the removal of improvised explosives seized along with 2,300 kilogrammes of illegal fireworks, officials said.
The blast blew the top off a bomb disposal truck and shot a fireball above the south Los Angeles neighbourhood last night.
At a press conference, Los Angeles Police Department Chief Michael Moore said that ten responding officers and seven civilians had non-life-threatening injuries from the explosion, which damaged homes and blasted out windows.
He added it was unclear why there had been a "total catastrophic failure of that containment vehicle."
"This vessel should have been able to safely dispose of that material," he said.
"Protocols were followed and pursued, but something happened in that containment vehicle that should not have happened and we don't know why."
Chief Moore said officers were responding to a call about illegal fireworks at a private residence when they found boxes of commercial-grade fireworks stored on the patio.
While removing the fireworks, the officers discovered additional, improvised explosives made of drink cans and simple fuses, he said.
Those devices were scanned and transferred to the specially-designed containment truck and detonated.
A resident at the house where the explosives were found, 27-year-old Arturo Cejas, was arrested and charged with possession of a destructive device.
Police are also looking to charge him with child endangerment after finding his 10-year-old brother on the same premises.
"We understand that this explosive material was acquired out of state and transported here for the purposes of reselling it to community members and neighbours for use during this Fourth of July holiday," Moore said.
Los Angeles has launched a fireworks buyback programme ahead of the holiday, hoping to stop the illegal sale and use of the banned explosives.
Last year, the LAPD said it seized more than four tonnes of fireworks.