Los Angeles police bomb technicians made major miscalculations in June when they detonated illegal fireworks improperly and caused a massive explosion that rocked a city neighborhood and injured 27 people in June, according to a report by federal investigators.

The 51-page report from the Bureau of Alcohol, Tobacco, Firearms and Explosives, released Tuesday, ruled out other possible causes, such as an equipment defect, for the June 30 blast in South Los Angeles. The Los Angeles Police Department also issued its own 53-page after-action report.

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The technicians overloaded a containment chamber with the illegal fireworks above the equipment’s safety rating after authorities were called to a South LA home for a huge stash of fireworks ahead of the Fourth of July. Preliminary findings previously said the bomb technicians’ errors were likely to blame.

The vessel exploded, and debris rained down on dozens of residences, businesses and vehicles. The ATF estimates that the catastrophic blast caused more than $1 million in damages to the neighborhood.

The LAPD report states that the agency’s bomb technicians do not undergo formal training to operate the containment vessel and merely have “on the job training” in its use. The technicians never weighed the homemade fireworks with a scale on June 30, and they grossly underestimated how much explosives they contained, investigators found. Officials also say that the agency’s evacuation procedures must be improved.

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Unlike other police departments, the LAPD does not transport explosives to a safe disposal site to detonate them away from homes and businesses. The department’s protocol on June 30 was to detonate the homemade fireworks in the containment vessel in the evacuated neighborhood if techs believed they were too unstable to be transported. Officials are now working to procure a detonation site.

“There have been instances in Los Angeles police history that has changed the course of this department forever,” LAPD Chief Michel Moore said Tuesday during a media briefing. “This is going to be one of them.”

The techs involved in the explosion have been pulled from bomb squad field duties and Moore does not expect them to return. They could still face discipline if an internal review finds they were negligent.

Officials revised the injury count to 27 in the newly released reports; authorities previously said it was 17 people. There have been 191 legal claims — the precursor to lawsuits — filed against the city; more than 40 have been settled.

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Many residents in the community remain displaced, and two elderly residents have since died. The Los Angeles Times reported that while officials have attributed their deaths to illness and natural causes, family members and activists contend that the explosion caused them stress that was a contributing factor in their deaths.

Fireworks are illegal to sell or possess in Los Angeles and in unincorporated areas of the county. The illegal fireworks were found at the home of Arturo Ceja III, who pleaded guilty in federal court last month to one count of transportation of explosives without a license. He also faces state charges.