WELD COUNTY, Colo. -- A flatbed truck crashed into a school bus carrying about 40 people in Weld County, Colorado on Thursday, CBS Denver reports. Investigators said the driver fell asleep at the wheel.

Investigators told CBS Denver the driver of the truck fell asleep while traveling southbound near Hudson, crossed into oncoming traffic and sideswiped the school bus. The bus rolled one-and-a-quarter times.

School district spokesman Casey Pearson tells KMGH-TV in Denver that 19 students were taken to the hospital with undisclosed injuries, and the bus driver was the most severely hurt.

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The Colorado State Patrol told CBS Denver that there were 40 people on board the bus, including students from Greeley Central High School and adult chaperones. It is unclear the age range of those on the school bus. About 10-15 students were transported to the hospital with various levels of injuries including broken bones.

"We had four that were what we call code red… that's a significant injury with the potential to be much more severe that it appears on scene. So they were transported immediately," said Stephanie Cooke with the Platte Valley Fire Protection District.

The Colorado State Patrol tweeted that none of the injuries appears to be life-threatening.