A tractor-trailer carrying 35,500 pounds of explosives overturned and exploded Wednesday, injuring four people and leaving a huge crater on Utah Highway 6 in Spanish Fork Canyon.

A motorist and a passenger in the truck's cab were among those transported to hospitals immediately after the 1:54 p.m. accident, Utah Highway Patrol Sgt. Todd Royce said.

LDS Hospital spokesman Jess Gomez said two person in critical condition were taken by medical helicopter to Utah Valley Regional Medical Center (search) in Provo. A third was brought in by ambulance in satisfactory condition.

A fourth person was transported by helicopter to University of Utah Hospital, spokesman Chris Nelson said.

The tractor-trailer driver was able to get out of his truck and warn other motorists away from the truck before the explosion. It was not immediately known if the driver was hurt.

The canyon about 60 miles south of Salt Lake City (search) was closed in both directions.

It was unclear how the accident occurred, Royce said. He said the truck was "pretty much vaporized" in the explosion and that that both lanes of the highway had been gutted by the blast.

The accident happened in an area about 20 miles east of the mouth of the canyon known as the Red Narrows.

Television coverage of the accident scene shows a large crater in the road and damage to nearby rail lines. Several small fires in the hills above the accident scene were believed to have been triggered by flying debris.

The rig with a 6-foot trailer from R&R Trucking of Missouri had just left commercial explosives maker Ensign-Bickford (search) at the mouth of Spanish Fork Canyon when the accident happened. The truck was headed to Oklahoma, company officials said. They wouldn't say what type of explosives the truck was carrying.

Hal Jaussi, an Ensign-Bickman manager, said the trucking company "met federal regulations for transporting explosives."

Trucking company controller Doug Greek said the company couldn't comment on the accident because they were "still trying to investigate everything that's happened."

"It's just crazy," Greek said.