New York (CNN) -- The driver of a commercial tour bus that crashed this week, killing four people and injuring 53, was charged with four counts of involuntary manslaughter, a prosecutor confirmed Friday.
Kin Yiu Cheung, 37, of Flushing, New York, was already was in custody on a reckless driving charge as the elevated charges were filed Friday.
Investigators blamed the wreck on driver fatigue.
The Virginia State Police Motor Carrier Safety Team ruled out any mechanical errors or malfunctions after inspecting the bus.
The U.S. Department of Transportation prohibited bus company Sky Express Inc. from operating interstate transportation services.
The Sky Express bus left Greensboro, North Carolina, at 10:30 p.m. Monday and was en route to Chinatown in New York City. It ran off I-95 and overturned early Tuesday in Caroline County, Virginia.
Sky Express is headquartered in Charlotte, North Carolina.
The transportation department's Federal Motor Carrier Safety Administration issued an unsatisfactory safety rating for Sky Express for violating multiple federal safety regulations for driver qualification requirements, drug and alcohol compliance, hours of service and vehicle maintenance.