A double-decker bus that may have made a wrong turn off the highway slammed into a low railroad bridge in the pre-dawn darkness Saturday in a wreck that killed four passengers and critically injured others, authorities said.

The Megabus was carrying 29 people, including the driver, when it rammed the bridge around 2:30 a.m. on the Onondaga Lake Parkway in Salina, a suburb of Syracuse in central New York.

The bus lay on its side after the crash. Four passengers and the driver, identified as John Tomaszewski, 59, of Yardville, N.J., remained hospitalized Saturday night, according to Onondaga County Sheriff Kevin Walsh.

One passenger was in critical condition and the other four, including the driver, were in serious condition, but were expected to survive, Walsh said.

Twenty passengers, including seven Canadian residents, were treated and released, said Don Carmichael, a senior vice president at Coach USA, which operates Megabus.

The bus was too tall and wasn't able to clear the low-hanging span, according to Walsh, who said he believed the driver made a wrong turn when getting off the interstate.

"Whether he saw the bridge, we don't know," Walsh said. "There were all kinds of warning signs at several locations, including flashing yellow lights."

It struck the bridge between two large signs warning that the clearance was 10 feet, 9 inches, photographs from WSYR-TV showed. The top level of the bus was obliterated in the front.

The driver had head injuries but was speaking to investigators, Onondaga County sheriff's deputy Herb Wiggins told The Post-Standard newspaper of Syracuse.

Police identified three of the dead as Kevin Coffey, 19, of Kansas, Ashwani Mehta, 34, of India, and Benjamin Okorie, 35, of Malaysia. The name of the fourth victim, a woman in her teens or early 20s, has not yet been released, said Walsh. He said there was no indication the driver had been drinking or using drugs.

The bus left Philadelphia at 10 p.m. Friday and was headed for Toronto with stops in Syracuse and Buffalo, said Carmichael.

Normally, the bus enters Syracuse on Interstate 81 and heads straight for a depot for a 30 minute rest stop, Carmichael said, but on this night, the driver left his usual route and was on a lakeside parkway that might have been unfamiliar.

"We don't know why," he said. Asked if the driver might have been lost, Carmichael said, "He had driven the route before."

The parkway and the transportation depot share the same exit off the interstate, and a driver who chose the wrong fork at the bottom of the ramp would find himself on the parkway. From there, it is only a short distance to the bridge, and in between there are no places for a driver to turn or pull off the road.

Branislav Nikolic, 25, and his wife, Maja Nikolic, 24, of Toronto, told The Post-Standard that they were sitting on the top level of the bus when the crash happened.

"I heard smashing, the sound of metal being broken and sliding on the other metal," Branislav said. "I heard my wife scream my name and at that point I realized we were flipping."

He said he and his wife got out easily because they were sitting in the middle of the bus.

Passenger Lee Veeraraghavan, 27, a University of Pennsylvania student who was traveling home to Toronto, said she heard people calling for help and moaning after the crash. She told The Post-Standard she was sitting in the back of the bus on the lower level.

"I just tried to get my bearings," Veeraraghavan said. "I just remember coming to in pain and a lot of broken glass under the bus, and there was a woman's legs on top of me."

After about 15 minutes someone on the bus pried open what she thought was a door and the passengers escaped, Veeraraghavan said.

Carmichael said the company began offering the daily trip on July 21 and the driver had been making it regularly since then.

The bus normally arrives in Syracuse at 2:10 a.m. and idles there until 2:40 a.m., so it was late getting in but not terribly so.

The crash shut down the parkway.

Carmichael said on Saturday afternoon that 17 of the passengers that had been released from the hospital were brought to a hotel to rest and decide whether they wanted to continue on their trip, or go home.

Megabus has operated the double-decker buses since 2007.

"This is a very, very unfortunate, horrific accident, and our primary concern right now is for the families and loved ones of the deceased. Our thoughts and prayers are with them," Carmichael said.

(This version CORRECTS Updates with names of three deceased, adds hometown of driver, corrects spelling of first name of Maja Nikolic, per officials.)