SALT LAKE CITY (AP) — Police are investigating a possible negligent homicide case against a Greyhound bus driver who passengers said passed out before the bus careened off a road in the Utah desert, killing a teenage girl and injuring 12 more people, search warrants made public Wednesday show.

Investigators tested the blood of driver Charles Edward Saunders, 62, after finding cold medicine at the scene and getting reports he slumped over the wheel before the crash, the documents state.

The results of the test were not released and no charges have yet been filed.

Greyhound declined to comment specifically on the case and about Saunders' record with the company. Greyhound policy requires ill drivers to book time off, following federal guidelines, said spokeswoman Lanesha Gipson. Drivers are also required to inform a manager if they take any kind of medication, she said.

Saunders was critically injured in the New Year's Eve crash and is now in good condition, Utah Valley Hospital spokeswoman Janet Frank said Wednesday.

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No attorney is listed for Saunders, who has a Nevada driver's license, and there was no immediate response to a message left at a publicly listed phone number.

Passengers told police that Saunders had been complaining of illness and not feeling well, the documents state. One rider told investigators that Saunders fell asleep and had to be awakened by passengers on the route to Las Vegas.

A person seated directly behind the driver told police he later had a coughing fit and slumped over the wheel shortly before the bus swerved off the highway and plunged into a ravine about 300 miles (483 kilometers) south of Salt Lake City, the documents state.

Summer Pinzon, 13, of Azusa, California, was killed and a dozen more people were hurt in the late-night crash along Interstate 70.

A total of 14 people were on board the bus. Most passengers escaped by climbing out of a rear window, and some flagged down a truck driver for help.