WELD COUNTY, Colo. — A pickup truck crashed into a school bus here on Thursday, causing the bus to roll and injuring most of the passengers, police said.

According to the Colorado State Patrol, the 2015 Ford truck was headed southbound on a county road when it drifted into a northbound lane and struck the 2016 school bus, which rolled one and a quarter times, ending up on its side. The crash occurred around 4 p.m.

Colorado State Patrol Trooper Gary Cutler told local news sources that the pickup truck driver, identified as 36-year-old William Carroll, fell asleep at the wheel. Carroll, who reportedly was not injured, was cited for careless driving.

The bus was returning from a field trip with students in Greeley-Evans School District 6’s Student Recovery Program, which helps at-risk youth catch up on credits. There were about 35 students and adults on the bus.

The Greeley Tribune reported that all but one of the school bus occupants were hurt in the crash, although the Colorado State Patrol had said on Twitter that about 20 of the bus occupants were transported with minor to serious injuries. School district spokeswoman Theresa Myers told The Greeley Tribune that the injuries ranged from bumps and bruises to back breaks and neck fractures.

Greeley-Evans Superintendent Deirdre Pilch said in a statement on Friday that the students and staff who were injured were taken to various hospitals in the area.

“Most were treated last night and released; others were kept overnight for observation,” Pilch said. “I am happy to report that at this time there does not appear to be any life-threatening injuries.”