The National Transportation Safety Board is investigating a tractor-trailer bus crash that happened Friday evening, killing four people and injuring more than a dozen others.

The incident happened around 9 p.m. along Interstate 35 south of Davis, Oklahoma, about 70 miles south of Oklahoma City near the Texas border.

A northbound tractor-trailer crossed the median, hitting the side of a southbound bus from North Central Texas College that was filled with 15 women’s softball players headed from a scrimmage against Southern Nazarene University, in Bethany, Oklahoma. The Texas college is located just south of the Oklahoma border in Gainesville.

Three of those on board died at the scene, a fourth death happening at a nearby hospital. All were between 18 and 20 years old. Two others remained at the hospital as of Sunday afternoon in fair to critical condition. Nine players, along with the coach who was driving the bus, were treated at various hospitals and released.

The driver of the truck, 53 year-old Russell Staley of Saginaw, Texas, was treated at a hospital and released. He was working for Quickway Transportation of Nashville, Tennessee, according to published reports, and was driving a 2013 Peterbilt.

The Associated Press reported on Sunday he told investigators that he may have been distracted by something in his truck just before the crash happened. However, investigators do not necessarily agree with his explanation of how the incident unfolded.

The sky was reportedly clear at the time of the crash and the road condition was dry, according to the Oklahoma Department of Public Safety. There's no indication he tried to make evasive maneuvers before the crash, according to investigators.

Both the bus and truck driver will undergo toxicology tests to determine if drugs or alcohol were a factor in the crash.

The Highway Patrol is investigating the crash as a homicide, and results will be turned over to a local prosecutor who will decide if criminal charges are to be filed.

A statement on the Quickway Transportation website offered condolences for the families of those killed and prayers for a full recovery of those hospitalized.

“We will continue to fully cooperate with Oklahoma State Patrol, the National Transportation Safety Board, and other authorities in their investigation of this tragedy.”

An AP review of Federal Motor Carrier Safety Administration records shows the company has not been involved in a fatal accident during the past two years, nor has it faced federal safety penalties in at least six years. Quickway has 327 trucks, 611 trailers and 436 drivers, according to the federal records.

Its driver out of service rate is 1.1% compared to a national average of 5.5% while its vehicle out of service rate is 7.4% compared to a national average of 20.7%.

Of Quickway’s 51 violations recorded in the past 24-month period, 13 were for failing to wear a seatbelt, the highest number of specific violations. There were two instances of using a handheld mobile device while driving. During that same period, there were no drug or alcohol violations.