The California Highway Patrol completed its investigation into last year's deadly collision between a FedEx vehicle and a tour bus, concluding that driver error – not environmental factors – caused the crash.

The crash happened April 10, 2014, around 5:30 p.m. local time along Interstate 5, about 100 miles north of Sacramento, involving a FedEx Freight truck and bus carrying Southern California area high-school students.

The CHP found that the FedEx driver had made an "unsafe turning movement," which caused the truck to plow through the median and into oncoming traffic, striking the tour bus and killing 10 people. The truck made no attempt to brake or evade the collision.

“Environmental factors, roadway conditions and vehicle maintenance were not the cause,” said Chief Ruben Leal of the CHP Northern Division. “The collision was caused – for unknown reasons – by the driver’s unsafe turning movement.”

It is believed that driver fatigue, an undetermined medical condition or sleepiness may have contributed, but the report indicated that could not be determined for certain. The investigation team did find some evidence to suggest that fatigue or sleepiness may have contributed to the crash.

The accident took place on a long straight section of road and the departure angle of the tires was consistent with fatigue-related collisions. Also, the driver was alone and didn’t attempt to avoid the collision.

An eyewitness had reported that the driver was slumped toward the window as he approached traffic.

Several lawsuits have been filed as a result of the crash, including negligence and wrongful death lawsuits against FedEx and the company that operated the tour bus. Initial reports mentioned the possibility of the FedEx truck having mechanical problems prior to the crash, but the CHP release made no mention of it in their final report.