Four people died after a semitruck hauling lumber lost control and plowed into vehicles on a crowded highway near Denver, triggering explosions and a fire so intense that it melted the roadway and metal off cars, authorities said Friday.

"It was crash, crash, crash and explosion, explosion, explosion," said John Romero, a spokesman for the Lakewood, Colorado, police department, describing the 28-vehicle chain reaction of blasts from ruptured gas tanks.

The truck driver, Rogel Lazaro Aguilera-Mederos, 23, of Houston, was arrested on suspicion of vehicular homicide as police investigate how the crash happened Thursday on Interstate 70.

Six people were taken to hospitals with injuries, but their conditions were unclear Friday.

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

Officials say Aguilera-Mederos, who sustained minor injuries, was headed down a hill when he lost control and slammed into traffic slowed because of a crash ahead of them involving a school bus and a tractor-trailer.

At a hospital, Leslie Maddox told Denver news station KUSA-TV that her car was among those tangled in the crash. She was receiving treatment for a broken arm and nose and credited two bystanders with preventing further injuries by pulling her from her car after the crash.

"I'm lucky to be alive," Maddox said.

There is no indication the crash was intentional or that drugs or alcohol were a factor, Countryman said, with investigators looking at whether the brakes on Aguilera-Mederos' truck were working.

Interstate 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 Rocky Mountains, where signs warn drivers to check that 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.

Workers cleared the burned and mangled wreckage from the highway and worked quickly to replace the top layer of burned pavement Friday. A burned tractor-trailer was barely recognizable as it was hauled away except for its size and its smokestack.

Aguilera-Mederos was scheduled to make his first court appearance Saturday to be advised of his rights. A judge also may consider bond. There was no information on whether he is represented by an attorney who could speak on his behalf.

Prosecutors have not filed formal charges, said Pam Russell, a spokeswoman for the Jefferson County district attorney's office.

Federal agencies are monitoring the investigation. The National Transportation Safety Board said its teams get involved when the agency sees an opportunity to issue new traffic safety recommendations.

The Federal Motor Carrier Safety Administration, which regulates and provides safety oversight for large trucks and buses, said it is working with state and local authorities to assist in any way it can.

___

Slevin reported from Denver. Associated Press writer Kathleen Foody in Denver contributed.