Oklahoma Highway Patrol Capt. Ronnie Hampton said the semi crossed the median and veered into the southbound lanes, striking the bus.
said three women from the team died at the scene and a fourth died at a hospital in nearby Sulphur.
said the team's coach, who was driving the bus, declined treatment at the scene and escaped without serious injury.
said 15 other bus passengers were hurt, as was the driver of the tractor-trailer, who was expected to survive.
The extent of the surviving team members' injuries wasn't known yet.
The front end of the team bus was virtually untouched, but the sides of the vehicle were heavily damaged.
Troopers and witnesses said the driver's side of the bus sustained the worst damage.
said that after the truck struck the bus it kept going and went off the side of the road.
"We knew we had a two-vehicle accident by what we saw at the scene. ...
But it took about an hour before we found the semi," he
The National Transportation Safety Board
said Saturday that it was sending a team of investigators to the site.
said the Highway Patrol's investigation should take several weeks.
"Right now, it's being investigated like a homicide," Hampton
said, adding that both drivers will undergo standard toxicology tests.
said details of the crash were still not known but "something happened to cause [the semi-tractor driver] to depart the roadway and impact the other vehicle."
said the accident will remain under investigation for several weeks.