A bus driver was killed and about 40 passengers injured Saturday morning when the bus crashed into an overturned tractor-trailer blocking Interstate 25 in central Wyoming.

Wyoming Highway Patrol Sgt. Stephen Townsend said the crash involving the Arrow Stage Lines bus going from Denver to Billings, Mont., occurred about 1:40 a.m. Saturday.

He said after the tractor-trailer crashed in the northbound lanes, a second tractor-trailer driver stopped and activated its hazard flashers.

Townsend said about three to five minutes later, the driver of the bus apparently didn't see the overturned tractor-trailer and ran into it. The bus driver's name has not been released.

Police said the bus remained upright. Nearly all 47 passengers were transported to medical facilities.

Townsend said the driver of the tractor-trailer that crashed was taken to a hospital, but that person's condition is unknown.

The crash was about four miles south of Casper on Interstate 25. Townsend said such crashes aren't uncommon in the state.

"We have these every now and again," Townsend said. "We have a lot of bus travel back and forth."