By Jonathan Allen

NEW YORK (Reuters) - A bus driver behind the wheel during an accident that killed 15 people in New York was overly tired and caused a crash that "did not have to happen," prosecutors said at his criminal trial on Thursday.

Ophadell Williams, charged with manslaughter and criminally negligent homicide, was driving a bus to New York City's Chinatown from the Mohegan Sun casino in Connecticut when it slid hundreds of feet, slammed into a guard rail and flipped over. Its roof was sliced off.

Of the 32 passengers, 15 died and 15 were injured, some severely, in the March 12, 2011 crash on Interstate 95 in New York City's Bronx borough.

Williams' attorney told the court that his client was simply unlucky and has told authorities from the beginning that the bus had been cut off by a semi-trailer. Injured in the accident, Williams nonetheless helped pull victims from the bus, attorney Patrick Bruno said.

"You'll get a picture of a hard working kind helpful man," Bruno said in his opening statement to the jury in State Supreme Court in the Bronx.

The National Transportation Safety Board ruled that the crash had been caused by driver fatigue and also said Williams had a terrible driving record. It found that he had had little sleep for days beforehand and criticized his employer, World Wide Travel of Greater New York, for lax oversight.

Williams was speeding, driving 78 miles an hour in a 50-mile-per-hour zone, and made no effort to slow down as he began to lose control, prosecutor Gary Weil said in his opening statement.

"At no time did the defendant ever apply the brakes, not for a second. Not the brakes, not the gear shift, nothing," he said.

"The evidence will show that he fell asleep or was so fatigued that he could no longer concentrate," he said.

Weil showed the jury maps of the accident site and photographs of the scene, describing how the roof of the bus was peeled off "like a sardine can."

"This accident did not have to happen," he said, adding that Williams could have notified his dispatcher or his company or stopped to rest.

"He did none of those things with 32 innocent souls behind him," the prosecutor said. "He knew the risks of driving while fatigued. He ignored those risks.

"He caused this crash to occur," he said.

Some relatives of the victims, many of whom were Chinatown residents, were in the Bronx courtroom.

"I reserve all my feelings for after the case. Right now I want to hear the facts of the case," said Florence Wong, whose father died in the crash.

World Wide Tours was shut down by federal regulators following the crash, and the NTSB has proposed tough new measures to crack down on bus operators, including fitting vehicles with devices limiting their speed and requiring companies to obtain 10 years of driving records from potential employees.

Williams told investigators he was run off the road by a semi-trailer, but the NTSB said it found little evidence that occurred. It found Williams had a driving record with 18 driving suspensions over two decades for incidents such as failing to obey a stop sign.

(Writing by Ellen Wulfhorst; Editing by Paul Thomasch)