Investigators know the driver of a Toronto-bound bus that slammed into a bridge took a wrong turn, but why he missed the warning signs remains a mystery.

Four people were killed and 20 others injured when the double-decker Megabus hit a railroad bridge on the Onondaga Lake Parkway in Salina, a suburb of Syracuse, N.Y., around 2:30 a.m. Saturday.

Don Carmichael of Coach USA, which operates Megabus, said the driver "obviously made a wrong turn" because at the time of the crash the bus had deviated from its usual route between Philadelphia and Toronto.

According to Carmichael, the 59-year-old driver had driven the correct route on numerous occasions.

Local police and Megabus are both probing the crash, Carmichael said, and investigators from both parties have already interviewed the driver, New Jersey resident John Tomaszewski.

Investigators say Tomaszewski was not intoxicated at the time of the crash.

"He had a very severe head injury so he's confused," Onondaga Sheriff Kevin Walsh said. "He's been co-operative, and as of last night he still didn't know how he ended up where he was."

Walsh said the bus was supposed to make a stop in Syracuse. But Tomaszewski turned the wrong way after leaving the highway and apparently never noticed the flashing signs cautioning against the low bridge ahead.

"Why he missed those signs, why he didn't see the bridge, those are things that are still unanswered," Walsh said.

Horrific scene

CTV's Omar Sachedina visited the crash site in Salina and reported Sunday that there are "signs everywhere that clearly delineate what the clearance requirement is, and this bus was quite a bit taller than this bridge."

Seven Canadians were among the 28 passengers on the bus, all of whom survived the crash with minor injuries.

Ray and Vickie Reed of Dundas, Ont. were asleep on the bus when it struck the bridge.

"I woke up thinking 'Oh my God, what just happened?' It was like a movie, but we were in the movie," Vickie told CTV News, recalling how the impact sent her flying over several rows of seats.

Reflecting on the resulting carnage, Ray Reed can hardly believe so many survived.

"I can't believe just four people died, it was just horrific."

Lee Veeraraghavan was also asleep at the time of the crash. She awoke lying on asphalt littered with broken glass, pinned under another passenger.

"While I was still on my back I looked over my shoulder and I saw a severed leg," Veeraraghavan told The Canadian Press Sunday from her parents home in Toronto, where she was recuperating from multiple bruises and a gash above her eye.

"I looked at it again to make sure that I saw what I had thought, and then when I realized what it was I turned away and didn't look at it again."

"That's when I realized the magnitude of the crash."

Veeraraghavan said passengers crawled past body parts, including a severed head and an arm, when they were leaving the site of the crash.

Victims identified

Four passengers were killed in the accident. All had been sitting on the upper level, near the front of the coach.

They have been identified as Kevin Coffey, 19, of Kansas, Ashwani Mehta, 34, of India, Benjamin Okorie, 35, of Malaysia and Deanna Armstrong, 18, of Vorhees, N.J.

Most of the passengers were sent to area hospitals and later transported home or to destinations of their choice.

Five people remain in Syracuse hospitals. One is in critical condition, while four others, including the driver, are in serious condition.

Police have taken statements from the passengers and will later conduct a full accident reconstruction to determine factors in the crash.

"At this point we just don't know until we've got all the facts together," Sheriff Walsh told CP.

Ultimately, a district attorney will make a decision on whether the case should go to a grand jury.

"There have been a lot of accidents along that stretch of road," said Walsh. "I believe this is the first fatal accident that had anything to do with the height of the bridge."

Megabus.com, which launched in April 2006, touts itself as a safe, low-cost bus service between 28 major cities in the U.S. Midwest and northeast.

The company was introduced in Canada in August 2009, operating routes in and out of Toronto, Kingston and Montreal.

With files from CTV's Omar Sachedina and The Canadian Press