A double-decker bus traveling off its usual route 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 27 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. Twenty-four people were taken to hospitals, most suffering from minor injuries, officials said. A handful remained hospitalized Saturday afternoon.

The bus was too tall to make it under the low-hanging span, said Larry Ives, supervisor of dispatch operations for the Onondaga County Sheriff’s Department.

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 crushed and partially peeled back in the front.

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

The dead included three men and a woman in her teens or early 20s, Sheriff Kevin Walsh told the newspaper. 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 Don Carmichael, a senior vice president at Coach USA, which operates Megabus.

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.”

He said the company began offering the daily trip on July 21 and the driver had been making it regularly since then. An investigation was under way to determine why he had left the highway, he added.

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, which runs along a lake not far from the intersection of Interstate 81 and the other major highway that runs through the city, Interstate 90.

Carmichael said on Saturday afternoon that 17 passengers had been released from the hospital and 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.