Investigators on Monday were trying to determine how a man found crushed to death by an elevator at a Bay Area Rapid Transit station managed to get into the shaft.

The man was discovered Sunday night after authorities responded to reports that a rider was stuck in an elevator.

The rider later told authorities that he heard noise while taking the elevator up at the bustling Montgomery Street station, transit police Lt. Michael Hayes told The Associated Press.

"The rider said he had been riding up to the concourse when he heard a crunching sound, followed by a scream," Hayes said. "Then the elevator stalled."

A man was later found dead on top of the elevator along with personal belongings that apparently included bedding, transit spokeswoman Luna Salaver said.

"We don't understand why anyone would seek refuge in an elevator shaft. It is disturbing, upsetting and highly unusual," Salaver said. "This has never happened before in our 40 years of service."

Authorities suspect the man was homeless, and it was unknown how long he had been inside the elevator, Salaver said. He had no identification when he was found.

BART ticket agents and police conduct sweeps inside stations prior to closing time. Typically, homeless and transient people camp overnight outside a station's gate to get away from the elements, Salaver said.

The incident is under investigation by transit police, and inspectors with Cal OSHA were expected to conduct a separate probe. The elevator could be back in service by March 20, Salaver said.

Investigators are trying to figure out how the man climbed into the elevaor shaft. There are only two ways to access the area where the man had been staying on top of the elevator, the ventilation shaft, which was still secure after the accident, and an elevator hatch in the ceiling of the car.

BART police are focusing their investigation on that hatch, which is usually secured with a lock.

There's also hope surveillance cameras inside the station might provide clues as to when the man snuck inside and how long he had been there.

It’s considered trespassing to stay in the BART station, but that doesn't mean BART doesn't have trespassers.

Last summer, BART police hired a crisis intervention consultant whose job it is to identify homeless individuals who repeatedly stay at specific stations.The hope is to better handle homeless individuals dealing with mental illnesses.