New York (AP) — The driver at the helm of a tour bus that crashed in a horrific accident last year, killing 15 passengers, knew the risks of fatigue and drove anyway despite a serious lack of sleep, prosecutors said Thursday as his manslaughter trial began.

Ophadell Williams had years of experience driving a bus, and simply should've known better, Assistant District Attorney Gary Weil said.

"This crime didn't have to happen," he said. "This is not a driver who maybe gets too little sleep. This is a professional, responsible bus operator."

The crash happened March 12, 2011, on Interstate 95 at daybreak as Williams was ferrying a busload of gamblers to Manhattan's Chinatown from the Mohegan Sun casino in Uncasville, Conn. The bus crashed just feet from a road sign that read: "Welcome to the Bronx."

The victims were mostly Chinese men and women over the age of 40 who were regulars at casinos.

"At the scene lay 13 people in a makeshift morgue, covered up, pulled from the wreckage," he said. "They died instantly, in a horrific crash caused by the reckless and criminally negligent behavior of that man — Ophadell Williams."

Williams, dressed in a grey suit, mostly looked down and wrote on a yellow legal pad with a red pen while Weil spoke. Defense attorney Patrick Bruno said outside court that Williams was not fatigued.

"I sincerely believe that he had as much sleep as many other people routinely have," he said.

Bruno will address the jurors later Thursday.

Aside from the 13 people who died at the scene, two died at a hospital and 15 others were injured, some severely. Weil catalogued how one man had his ear torn off and others suffered bone fractures. The bus struck a guard rail, then toppled over and hit a signpost that opened the top like a sardine can before skittering down a hill to a stop.

One victim "put his arms over his head to protect his head, and both arms were ripped off," Weil said.

Williams pleaded not guilty to manslaughter and criminally negligent homicide. He is being held in Rikers Island because his family cannot post $250,000 bail. His wife didn't speak to reporters outside court Thursday. There were few victims' family members in the audience.

The National Transportation Safety Board said in June that the accident was probably caused by driver fatigue, and a bus company that provided too little safety oversight. It stopped short of saying Williams had fallen asleep.

Williams said that a tractor-trailer had cut him off, and that's why he swerved and hit the guardrail. But investigators could find no indication that had occurred. His attorneys have said he was awake and alert, and he's wracked with guilt over the crash, but not guilty of any crimes.

Weil said witnesses will testify that there was no tractor-trailer, and that Williams was driving erratically for miles before the crash — and didn't break when he hit the pole. The bus was traveling at speeds up to 78 mph in a 50 mph zone until seconds before it ran off the road.

Williams worked for World Wide Tours of Greater New York. Federal regulators shut down the bus operator after the accident for safety violations. Williams had not turned in any driver's logs while working for the company as required by federal safety regulations, yet World Wide took no action, federal investigators said. He had been being fired by two previous employers and had 18 suspensions of his driving privileges over two decades when he was hired by World Wide.

But the bus company won't face any criminal charges related to the crash, Bronx District Attorney Robert T. Johnson said.

New York state has stepped up inspections of tour buses since the crash. Dozens of buses have been taken out of service after police found problems with logbooks, licenses or equipment.

But on July 4 this year, another casino bus went out of control 2 miles up the road from the March 2011 accident, sending 24 people to the hospital with minor injuries. The bus was on an early morning route from Foxwoods Resort Casino in Connecticut to Chinatown in Queens in the rain when it struck a center median barrier on Interstate 95 in New Rochelle, veered right over three lanes, then slid about 500 feet along an outer barrier before stopping.

New York State Police said that it appeared the driver had been going too fast for the wet conditions.