LOS ANGELES (AP) — A massive explosion rocked a Los Angeles neighborhood as homemade fireworks were being destroyed by a bomb squad, leaving a trail of destruction, injuries, and questions in its wake as the July Fourth holiday approaches.

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

According to the Associated Press, the investigation into how the explosion occurred could take days.

The AP reported the material was being carried inside "a spherical containment vessel" while on a tractor-trailer when it exploded.

The explosion was so powerful that it tore the rig apart.

The illegal fireworks were discovered in a South Los Angeles neighborhood and were being transported away in what was considered a safe operation.

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. Or both.