Officials have confirmed no-one was sat at the controls of an empty train when it crashed into another train in Chicago, leaving dozens of commuters injured.
Authorities have so far been unable to establish how the train began moving. Investigators are now trying to determine if the vehicle started moving by itself or if someone sent it on its course intentionally.
Video footage shows that nobody was driving the 4-car Chicago Transit Authority train as it veered the wrong way towards the train parked at the Harlem Avenue station, 10 miles west of Chicago.
Over four dozen passengers on the parked train were treated for minor injuries and released from hospitals, authorities said.
Chicago Transit Authority spokesman Brian Steele said investigators are examining the video, the signalling systems and other data to determine what happened, adding that he had never heard of such an incident before. The footage is being turned over to the National Transportation Safety Board who are now leading the investigation, according to CTA spokeswoman Lambrini Lukidis.
The train started its journey beyond the last station on the west end of the Blue Line that runs from the western suburbs, into the city and out to O'Hare International Airport.
Kelly said the train was parked perhaps a quarter mile beyond the Forest Park station, in a spot where it could not be seen by any commuters or anyone else, including CTA workers, inside the station. He said that it wasn't until a CTA worker spotted the empty train roll through the station that there was any indication that anything was wrong.
That CTA supervisor frantically tried to contact the moving train. A few minutes later, the motorman of a westbound train that had just pulled into the Harlem Avenue station spotted the train coming at his.
"He pops the door open and walks on the platform to see where the train is going and tries to call it," said Kelly. When he got no response, said Kelly, the motorman shouted at the riders in the first car to "brace yourself" for a crash.
"We were stopped at Harlem and then we hear like a big boom sound and everyone started flying out of their seats," said passenger Lyneisha Fields. The 18-year-old said she was taken to a local hospital's emergency room because she hit the back of her head on a metal bar in the train.
The crash left a section of one train crushed.
Firefighters from a dozen local fire departments responded, Calderone said. Calls seeking comment from the Forest Park fire department were not immediately returned.
Additional reporting by Associated Press