New York (CNN) -- Four people were killed early Saturday when a double-decker Megabus slammed into a railroad overpass and turned on its side in Salina, New York.
Authorities said 25 passengers were transported to the hospital. By Saturday evening, five were still in the hospital, while one person was injured critically, said Onondaga County Sheriff Kevin Walsh.
The owner of the bus said it was off its designated route, but did not explain why.
"Obviously the driver didn't see the signs or didn't realize the height of his bus, that the double-decker bus wouldn't fit under that bridge," Walsh told CNN affiliate YNN.
Walsh said authorities suspect the driver missed a turn when a road split. A flashing yellow light alerts drivers of a low bridge, he added.
The bus, part of a fleet of low-cost express buses, was on its way to Toronto, Canada, from Pennsylvania. The accident occurred around 2:30 a.m.on the Onondaga Lake Parkway in Salina, a Syracuse suburb.
"Our immediate thoughts and concern are with those who have lost their lives or been injured, as well as their families and loved ones," a Megabus spokesman said. "Safety is our absolute priority and we will be carrying out a full investigation into the accident as well as assisting the police and other authorities in any way that we can."
The dead were people from India, Malaysia, New York and a Midwestern state, Walsh said.
Don Carmichael, senior vice president of Coach USA, which operates Megabus, said the company was tending to the needs of those on the bus.
"The key thing is to ensure that our customers and passengers are looked after," he told reporters.
CNN's Jackie Castillo contributed to this report