The tractor-trailer that collided with a bus and killed four members of a Texas community college women's softball team drove straight through an interstate median without any signs of braking or trying to avoid the collision, federal investigators said Sunday.

The truck was northbound on Interstate 35 near Davis, Oklahoma, Friday night. As the roadway made a gentle curve to the right, the truck instead continued straight. It traveled about 820 feet (250 meters) through the median and struck the southbound bus carrying 15 members of the North Central Texas College team and driven by their coach, said Robert Sumwalt, a member of the National Transportation Safety Board.

Even after smashing into the driver side of the bus, the truck continued another 300 feet (90 meters), crossing the southbound lanes and barreling through and uprooting trees, Sumwalt said.

"Basically, the general trajectory from the beginning of the roadway departure all the way to the collision and then onward into the trees was basically a straight line," Sumwalt said.

Asked if that suggested the driver was sleeping or otherwise indisposed, Sumwalt said: "That's going to be the million dollar question."

"Was it something with the vehicle or was it something with the operator of the vehicle?" he said.

Sumwalt said investigators found no problems with the truck's brakes, but were still trying to download the truck's data recorder, which could show if the brakes were applied. Investigators had interviewed one of the softball players who remained in the hospital and hoped to interview the other later Sunday.

The Oklahoma Highway Patrol, which is conducting the criminal investigation, said Sunday that the truck driver, Russell Staley, 53, of Saginaw, Texas, told investigators he was distracted.

"He (Staley) said he was distracted by something in the cabin," said Oklahoma Highway Patrol Capt. Ronnie Hampton, who declined to say what the distraction was. He said that investigators do not necessarily agree with Staley's explanation. No charges were pending against Staley.

Besides the deaths, a dozen people were injured in the crash although all but two had been treated and released. Oklahoma University Medical Center said Bailey Buchanan, 18, was upgraded to stable condition Sunday from critical earlier. Rachel Hitt, 19, remained in fair condition at Norman Regional Hospital, a spokeswoman said.

The team was returning from a scrimmage against Southern Nazarene University in Bethany, Oklahoma.

The four players killed were identified by the Oklahoma Highway Patrol as Meagan Richardson, 19, Brooke Deckard, 20, Katelynn Woodlee, 18, and Jaiden Pelton 20. All were from towns in North Texas.

A prayer vigil was scheduled Sunday night at the college's Gainesville campus, which is just south of the Oklahoma-Texas border.

Staley was driving for Quickway Transportation of Nashville, Tennessee, which issued a short statement on its website Sunday.

CEO William Prevost said the company was fully cooperating with the investigating authorities.

"Trusting in God's grace, we hope to one day join the college in properly memorializing these ladies lives and their legacy," the statement said.

The company statement did not refer to the driver of the truck.

According to federal data, Quickway's safety record has been good in recent years.

Records with the Federal Motor Carrier Safety Administration show that Quickway had not been involved in a fatal accident in at least the past two years through Aug. 22. The company had not faced federal safety-related penalties in at least six years, based on available records. Quickway was registered with 327 trucks and 436 drivers.

Nationally, crashes involving a large truck or bus killed 4,281 people in 2013, little changed from the number of deaths in each of the previous three years.