RICHMOND, Va. (CBSDC/AP) — The National Transportation Safety Board has released hundreds of documents in its investigation into a bus crash on Interstate 95 in Virginia that killed four people and injured dozens.

The bus bound for New York overturned about 30 miles north of Richmond on May 31, 2011.

Four people were killed in when the Sky Express operated bus returning to Chinatown from North Carolina swerved off the road and overturned on I-95 in Virginia.

Court records show driver Kin Yiu Cheung admitted to police that he fell asleep at the wheel. He is set to go to trial in Caroline County later this month on four counts of involuntary manslaughter. Dispatcher Zhao Jian Chen is set to face the same charges in October.

The docket includes about 630 pages of interview summaries, investigative reports, more than 65 photographs and other documents.

Officials say the likely cause of the crash will be made available when the investigation is complete.

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