(Reuters) - The driver of a semi-trailer truck that crashed into a bus, killing four members of a woman's college softball team from Texas, told police he was distracted just before Friday's accident on an Oklahoma highway, police said on Sunday.

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 softball team from North Central Texas College.

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

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.

All team members were transported to area hospitals with injuries from minor to severe, medical officials said. The coach refused treatment at the scene.

The truck driver was also hospitalized briefly and released, police said.

The crash investigation was being conducted like a homicide, and evidence will include toxicology reports, a reconstruction of the scene and witness statements, Hampton said. The investigation could take up to 10 weeks, he said.

The National Transportation Safety Board (NTSB) was also investigating. Both the NTSB and Oklahoma Highway Patrol were expected to hold separate news conferences Monday.

A vigil for the students was scheduled for this Sunday evening.

"This is the most traumatic event that NCTC has had in its 90 years of history," said the college's president, Brent Wallace.

He described his college, and especially its sports teams, as being tight-knit.

In response to the crash, games scheduled for the school's sports teams were canceled over the weekend.

(Reporting by Victoria Cavaliere in Seattle; Editing by Mary Wisniewski and Eric Walsh)