COEUR d'ALENE - Rocks in the westbound lanes of Interstate 90 led to a four-car pileup shortly before noon on Wednesday.

Witnesses described some of the rocks as small boulders -three and half feet wide and two feet tall - and said one caused a Chevy truck to go airborne after hitting it near milepost 26 on Fourth of July Pass.

Witnesses suspect the rocks fell from the mountainside onto the road.

"I caught it out of the corner of my eye, this truck jumping up and then 'kaboom,'" said Brad Hash, who was driving next to the red SSR truck with an Oregon license plate that hit the rock in the left lane and crashed into Hash's Nissan. "I was trying to maintain control but he's taking us right off the road."

The two cars slid about 100 feet side-by-side before crashing off the north shoulder together.

"Then the airbag hits and I'm smelling this stinky stuff," Hash said. "I was a little shook up. Before I really knew what was going on we were halfway through the accident, it happened so fast."

The truck driver's identity or condition were not available Wednesday afternoon. He was taken to Kootenai Medical Center after the crash, witnesses said. Idaho State Police are investigating.

Idaho Transportation Department officials said the department hasn't heard of falling rocks near that stretch of highway before. ITD is in charge of maintenance in their rights of way, but falling debris and rocks are more common in the spring after the ground has thawed.

"Generally speaking it's not an issue in that area," said Wally Brown, assistant ITD maintenance engineer.

Areas that have lots of problems put up barriers or screens as a preventive measures. Rebuilt in the 1980s, the slopes in that stretch of highway were terraced to prevent that type of debris falling, although it does not have screens.

Brown called the accident "an uncommon occurrence" and that it's difficult for the department to inspect every piece of land near roadways due to resources.

"We just don't have a crew that can go out and scour every part of the mountainside to see what it's like," Brown said.

The initial accident led to a second accident around 100 feet to the east when one car rear-ended another car that had slowed as it approached the scene.

A driver from that accident was also taken to the hospital, according to witnesses, although the driver's identity or condition couldn't be confirmed.

Witnesses said rocks and debris covered both sides of the highway.

"There were rocks all over the place," said Phil Korell, who came upon the accident on his way to Seattle. "It was all a field of dust."

After the wreck, witnesses removed the rocks and debris from the road and helped the driver of the truck.

"He was shaken," said Jennie Rodriguez. "He didn't know what happened. He didn't remember anything. He thought he hit all of us. That's when he kept asking everyone around his car, 'Did I hit you, Did I hit you?' He didn't know until later when we sat him down and were trying to take the blood off him, did we tell him he hit a rock."

David Tremaine, who was riding with Hash from Missoula, Mont., when the Nissan was struck by the truck, remembers trying to navigate through the rocks before seeing the red truck in the left lane.

"Suddenly we get slammed and we see the flash of red," he said "I was just holding on thinking, man, we got to slow down."

Both Hash and Tremaine were uninjured.

"It could have been a lot worse," Tremaine said.