Four people were killed near Denver after an out-of-control tractor-trailer roared down a hill and into a rush-hour traffic jam Thursday, incinerating cars and semi-trucks in a chain-reaction fireball on busy Interstate 70.

The 23-year-old driver of the flatbed truck was arrested on suspicion of vehicular homicide, authorities said. The suspect, Rogel Lazaro Aguilera-Mederos of Texas, is expected to appear in court Saturday.

At least 24 cars and four semis were involved, many of them destroyed. Describing a scene of “significant, just unbelievable” carnage, Lakewood, Colo., police spokesman Ty Countryman said some bodies were still in the wreckage hours later, with darkness and the smoldering remnants of the vehicles hampering recovery efforts.

The gruesome aftermath left a blazing, apocalyptic stretch of highway in Lakewood, with towering orange flames belching columns of black smoke.

One truck was torched through to the engine block. Cars sat in ashen ruins. A pile of lumber from a truck was scattered about the roadside, much of it untouched by flames.

Another collision had set the stage for the catastrophe on I-70, authorities said. It was shortly before 5 p.m., and a collision up the road had brought traffic nearly to a standstill.

The truck lost control and slammed into the traffic jam, “causing a very big chain-reaction crash that ignited and started a fire,” Countryman said, describing the scene as “devastation.” Preliminary evidence does not point to a deliberate collision, officials said.

In addition to the four fatalities, six people — including the tractor-trailer driver — were taken to the hospital for treatment, Countryman said. The driver was later taken into police custody.

Investigators were still trying Friday to determine the cause of the collision and whether mechanical failures played a role, he said. It was unclear whether what the truck was carrying contributed to the size of the blaze, and Countryman said no signs pointed to alcohol or drugs as a factor.

Video taken around the time of the crash showed a semi barreling down the interstate at high speed as traffic around the truck lurched forward.

“All the cars were stopped; it was a traffic jam. And then this semi-truck . . . sped by, going it must have been over 60 miles per hour, 80 miles per hour, out of control down the emergency lane,” said Joshua McCutchen, whose dash cam seems to have captured the truck zooming past him.

McCutchen posted video of the semi along with scenes of the aftermath on his YouTube channel, Burger Planet.

The truck “scared the bejeebies out of me,” McCutchen said in a second video he posted on his channel. “And then I just see this huge explosion and fire and smoke coming from the other side of the overpass.”

Firefighters extinguishing the flames. (West Metro Fire)

Countryman said he was aware of bystander videos, including one that appeared to show a semi blowing past a runaway ramp, but that authorities had not confirmed it was the same tractor-trailer. The driver was the apparent wood hauler, he said.

The Colorado Department of Transportation said a bridge above the crash suffered damage but will be structurally sound. The asphalt below, however, was destroyed in the inferno, which melted aluminum, said Joshua Laipply, an agency spokesman.

Fire “can turn pavement into rubble, essentially,” Laipply said.

Traffic remained closed in both directions Friday near the scene, police said, as the investigation continued.

West Metro Fire personnel initially raced to the scene to extinguish the flames and dispatched drones to assess the area. Even after the fire had been put out, the department said on Twitter, heat signatures still glowed bright red and orange on an infrared camera.

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