LAKEWOOD, Colo. (AP) — Four people died after a semi-truck hauling lumber plowed into vehicles on a crowded section of Interstate 70 near Denver, causing a fire so intense that it melted the roadway and metal off of cars, authorities said Friday.

Authorities had to wait until daylight to confirm the death toll from Thursday afternoon’s 28-vehicle pileup because of the devastation caused by the fire.

Lakewood police spokesman John Romero described it as a chain reaction of crashes and explosions from ruptured gas tanks. “It was crash, crash, crash and explosion, explosion, explosion,” he said.

Six people were taken to hospitals with injuries. Their conditions were unclear Friday.

“There is just a bunch of debris from this crash that took place. The carnage was significant, just unbelievable,” another department spokesman, Ty Countryman, said.

The driver of the truck suspected of causing the crash, Rogel Lazaro Aguilera-Mederos of Houston, has been arrested on suspicion of vehicular homicide, police said.

Officials say 23-year-old, who sustained minor injuries, was headed down a hill when he slammed into traffic slowed because of a previous crash ahead of him on I-70. There is no indication the crash was intentional and that drugs or alcohol were a factor but investigators were looking at whether his truck’s brakes were working, Countryman said.

I-70 is Colorado’s vital east-west highway that connects the mountains with the plains and traffic has grown worse as the state’s population has boomed. The crash happened just after the highway descends from the mountains, where signs warn drivers to check to make sure their brakes are cool and working after traveling down the steep grades.

There are also ramps on hills off the sides of the highway for trucks that lose their brakes so drivers can exit and slow down before hitting other vehicles.

Pam Russell, a spokeswoman for the Jefferson County district attorney’s office, said Aguilera is scheduled to make his first court appearance on Saturday to be advised of his rights. A judge also may consider bond during that initial advisement hearing. There was no information on whether he is represented by an attorney.

Russell said formal charges have not been filed by prosecutors.

A spokesman for the National Transportation Safety Board said the federal agency is monitoring local authorities’ investigation into the crash but it is not involved. Peter Knudson said the NTSB’s teams get involved when the agency sees an opportunity to issue new traffic safety recommendations.

He said the agency opens investigations into between 15 and 20 of an estimated 8 million traffic accidents that occur in the U.S. each year.