A bus driver who fell asleep at the wheel on a Virginia interstate, causing a deadly early morning crash, was found guilty today of four counts of involuntary manslaughter, a county court clerk said.

In addition to four killed, 49 other passengers were injured around 5 a.m. on May 31, 2011, when a Sky Express Inc. motorcoach drifted off Interstate 95 near Richmond, Virginia, struck a cable barrier, spun around and then overturned.

The driver, Kim Yiu Cheung, was slightly injured in that crash and refused medical treatment, officials said.

Ray Campbell, a clerk for Caroline (County) Circuit Court in Virginia, said Cheung is scheduled to be sentenced January 23, 2013.

This summer, the National Transportation Safety Board concluded the deadly accident could be traced to the driver's "acute sleep loss."  It also blamed the bus company for not monitoring drivers' rest and sleep activities and a federal agency that oversees motor carrier companies for allowing the company to continue running despite known safety issues.