The van involved in the single-vehicle accident Monday that killed eight Utah State University students and an instructor was likely going between 95 and 100 mph when it crashed, the Utah Highway Patrol said Thursday.

"There is no good explanation," USU spokesman John DeVilbiss said. "It's uncharacteristic with a driver with a commercial driver's license. Information like this, it's unfathomable."

The accident investigation is also definitely showing that USU instructor and academic adviser Evan Parker, 45, was driving the van when it crashed, UHP trooper Jeff Nigbur said, noting that the fatal injuries to the popular agriculture professor are consistent with a person in the driver's seat during a rollover.

Both UHP and National Highway Safety Transportation Board investigators maintain that none of the 11 occupants was wearing a seat belt, despite claims by the families of the survivors that using seat belts was routine for their sons.

Survivors Jared Nelson, 22, Woodburn, Ore., and Robert Petersen, 21, Tremonton, have similar diagonal bruising across their chests that their parents believe are consistent with injuries a seat belt could have caused during the extremely violent crash.

Nelson is comatose and remains in critical condition at Ogden Regional Medical Center. Peterson is in serious condition at McKay-Dee Hospital.

UHP officials will continue their investigation of the crash, which includes examining the tires and other factors that could have caused the van to roll four times.

"Doing 95 to 100 mph will affect things on your vehicle. There are a lot of forces that go into it," Nigbur said. "Tires, they are speed-rated for certain speeds. Maybe some tires aren't made to go that fast. There are just a lot of different facts that go into getting an answer."

The blown tire, DeVilbiss said, had been on the vehicle since 2000 and had 16,000 miles of wear, he said.

"We're pretty confident that this tire was safe," he said the night of the accident. "They are very careful about maintaining these vehicles." The 1994 Dodge Ram van passed a state vehicle inspection on June 8, he said.

In May, the NHTSA warned of the dangers of driving on tires that are not properly inflated, which increases the risk of rollover.

The 15-passenger vans appear to have the highest risk of rollover under heavily loaded scenarios on high-speed roads, according to the NHTSA, which issued a 2004 report recommending that vans "should be driven with the utmost care while driving on high-speed roads."

Former USU President Kermit Hall recalled Thursday the April 2001 accident in which one of the school's vans crashed in a snowstorm in Wyoming. Six students were injured.