LOS ANGELES (AP) — Authorities say it may take days to determine why a cache of homemade fireworks exploded as it was being destroyed by a bomb squad in Los Angeles, injuring 17 people.

The blast Wednesday evening destroyed an armored tractor-trailer rig, damaged nearby cars, and shattered windows in homes. Most injuries, however, were minor.

The blast occurred after police acting on a tip found tons of commercial-grade fireworks at a South Los Angeles home.

Most were trucked away to a safe location but around 240 improvised explosives were considered too unstable to move and were placed in the armored truck to be detonated on site. Instead, the rig exploded.

Seventeen people were hurt — 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.

Experts say the explosion was highly unusual. The blast could have been the result of human error or a defect in the equipment like a micro-fissure that has grown with time and use.