There was no sign the driver of a truck that hit a bus, killing four college softball players from Texas, tried to avoid the collision after he left an Oklahomahighway traveling at about 70 miles per hour, U.S. officials said on Monday.

The 18-wheel tractor trailer was headed northbound late Friday on Interstate 35, a major north-south corridor, when driver Russell Staley, 53, veered across the median and struck a bus carrying the women's team from North Central Texas College.

"There was no swerving," National Transportation Safety Board member Robert Sumwalt told a briefing in Oklahoma. He said an investigation of the scene showed the truck was approaching a curve in the road and continued straight, taking it into the bus's path.

Staley told investigators he was "distracted" inside the truck's cab just before the accident, but the validity of the statement was still being assessed along with other evidence from the scene,Oklahoma Highway Patrol Captain Ronnie Hampton said.

The NTSB has not yet spoken to Staley, Sumwalt said.

Three members of the "Lady Lions" softball team were pronounced dead at the scene, and a fourth player died after being taken to a hospital, authorities said.

Fifteen student-athletes were on the bus, which was driven by their coach as they made their way home after a game at Southern Nazarene University, about 150 miles north of their college in Gainesville, Texas.

"This is a tragedy," Sumwalt said.