(CNN) At least four people died when the driver of a semi plowed into stopped traffic along Interstate 70 near Denver, and that driver is being held on four counts of vehicular homicide, police said Friday.

The runaway semi hit several cars and trucks in Lakewood on Thursday, igniting a fiery crash. Twenty-eight vehicles were involved in the pileup: four of them semis and 24 cars, Lakewood Police Department Agent Ty Countryman told reporters.

Countryman said the driver of the semi is Rogel Lazaro Aguilera-Mederos, 23. He has a commercial driver's license out of Texas, Countryman said. Aguilera-Mederos is cooperating with investigators, he added.

Aguilera-Mederos was behind the wheel of a flatbed truck loaded with lumber and was injured in the crash, but not seriously hurt, Countryman said.

According to the Jefferson County Sheriff's inmate roster, his next scheduled court appearance is listed as Saturday at 3 p.m. ET in Jefferson County court.

The suspect in custody is Rogel Lazaro Aguilera-Mederos

The suspect's brother, Daniel Aguilera, said Rogel is Cuban and is a permanent resident of the United States. Also a truck driver, Aguilera said he spoke to his brother after the crash and Rogel told him his brakes failed.

Countryman said there's no evidence of drugs or alcohol in the crash.

When asked whether there were any mechanical issues with the truck, Countryman said it is something his department is looking into, but he believes even if there were a mechanical issue, vehicular manslaughter charges would still stick.

The semi hit cars stuck in a jam due to evening rush hour and a prior crash farther ahead, Countryman said. At least six people were hospitalized with injuries after the crash, according to the Lakewood Police Department.

Video from the scene showed towering orange flames and piles of lumber scattered across the highway.

"We do know that I-70 eastbound traffic was at a standstill or close to a standstill because of a crash way out ahead of it," he said at a news conference. "The semi was eastbound and ended up colliding with the ... cars that were stopped because of the accident ahead. That's all we know or we are releasing at this point."

'Startled and in shock'

Josh McCutcheon, who caught the crash on video, told CNN sister networki HLN he had never seen such devastation. He's doing fine now, he said, but he was "startled and in shock" when he saw the deadly pileup, the explosions and the plumes of smoke.

"Oh, my God," he shouted when it happened.

McCutcheon praised a "good Samaritan" for pulling people out of cars.

"That's a true hero," he said of the man, who had been panhandling in the area.

Video from the scene showed towering orange flames and piles of lumber scattered across the highway. By Thursday night, the area was littered with smoldering ruins.

Darin Barton was panhandling near the scene when the crash happened. He told CNN affiliate KDVR that when the cab of the truck rolled over and flames ignited, he quickly dropped the sign he was holding and ran toward the cars.

"It just caught on fire. And I just dropped my sign, took off running," Barton said. "As soon as I [saw] flames, I headed under the bridge, grabbed three or four people out of a couple cars."

The fire was extinguished, but glowing embers and hot spots prevented investigators from immediately getting to the crashed vehicles. Investigators are working to match who was in each car, police said.

It's unclear what made the vehicles ignite, Countryman said, adding that he does not know what was in the semis other than the one that appeared to be carrying lumber.

"This is looking to be one of the worst accidents we've had here in Lakewood," he said.

The Denver West Parkway bridge over I-70 sustained significant structural damage from the fire, authorities said. The portion of I-70 affected by the crash is closed overnight in both directions.

Lakewood is about 10 miles from Denver.