SALT LAKE CITY (AP) — Police documents show cold medicine was found at the scene of a fatal Greyhound bus crash where passengers say the driver fell asleep and was awakened before the bus left the road in the Utah desert.

The Utah Highway Patrol and Greyhound said Wednesday the case remains under investigation. The driver’s name has not been released.

Police have said the crash was caused by a driver medical problem or fatigue.

Search warrants affidavits show passengers reported the driver was awakened by passengers after falling asleep en route to Las Vegas. Passengers said he later had a coughing fit and slumped over the wheel right before the bus went off the road.

Investigators state cold medicine was visible at the scene where 13-year-old girl died and 12 people were hurt on New Year’s Eve.