LOS ANGELES (AP) — 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.

Federal investigators on Tuesday released a report saying 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.

Fireworks are illegal to sell or possess in Los Angeles.

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.