The driver of a tour bus that crashed in March and left 15 people dead pleaded not guilty to manslaughter Thursday as details surrounding his past came to light. NY1's Zack Fink filed the following report.

Ophadell Williams, 40, faces multiple manslaughter charges and up to 15 years in prison.

The driver pleaded not guilty in a Bronx court Thursday and claimed he lost control of a World Wide Travel tour bus heading back to New York from a Connecticut casino trip after it was sideswiped by a tractor-trailer.

However, surviving passengers said Williams was asleep behind the wheel when the bus slammed into a sign off Interstate 95, slicing its roof off.

"As tragic as that accident was, we believe it is compounded by the fact that it was avoidable," said Bronx District Attorney Robert Johnson.

Investigators say Williams was driving the bus as fast as it could go, and well above the speed limit.

They also found no indication another vehicle was involved.

"From Connecticut on down to the Bronx, the defendant was driving recklessly, speeding, weaving in and out of lanes — all classic symptoms of sleep deprivation,” said Bronx Assistant District Attorney Gary Weill.

Williams' defense attorney claimed that the crash was an accident.

"It’s an accident case. That’s really what happened here. My client gives an entirely plausible explanation for what happened to him that day. We haven’t heard at all of any other contributing factors such as alcohol, narcotics, drugs, medications,” said Steven Seener, Williams’ attorney.

Williams is being held on $250,000 bail.

Meanwhile, Williams's criminal history is shining light on flaws in the state's licensing process.

Investigators say because Williams used different names and submitted false or misleading information to law enforcement, he was able to conceal aspects of his driving and criminal history.

In 1992, Williams was sentenced to three to nine years in prison for a Brooklyn homicide.

Six years later, he served four years in prison for stealing money from the city's Police Athletic League.

He had been driving on a suspended license since 1998, and at the time of the crash, he had three suspensions to his driver’s license under an alias, Eric Williams.

According to a report by the State Inspector General, the Department of Motor Vehicles was unable to connect Williams to his alias.

The inspector general made several recommendations, including improving communication between the DMV and law enforcement, checking drivers' histories under their previous names and providing commercial bus companies more information about driver history.

In response to the report, the state DMV said in a statement it is receptive to the ideas and has undertaken new initiatives like facial recognition technology.

Williams is due back in court on December 1.