TONOPAH, Ariz. — Authorities in Arizona are investigating a crash that killed four people and left two in critical condition after several tractor-trailers collided with nearby cars on a highway.

Firefighters hosed down charred cars as billowy smoke covered the roadway after seven vehicles caught fire from crashing on an Arizona interstate Saturday afternoon. The wreck caused hourslong road closures.

Six more people were injured and transported to hospitals, five of whom were treated and released, according to a statement from the Arizona Department of Public Safety. The fatal crash occurred near Tonopah, about 50 miles west of Phoenix.

At least two tractor-trailers were strewn off to the shoulder of the highway and multiple cars sat clumped together in the middle of lanes covered in soot and ash.

Authorities were investigating whether an initial collision caused by poor visibility from “blowing dust” led to a series of secondary crashes.

More than 20 units from three different fire departments responded.

— The Associated Press