Sources within the Chicago Transit Authority said Monday morning investigators are looking into the possibility that one of the CTA Blue Line trains involved in a collision in Forest Park might have been taken from the yard without authorization.
Forest Park mayor Anthony Calderone said the crash site is being treated as a crime scene.
"Until it's ruled otherwise," he said.
Chicago Transit Authority officials, however, said they do not believe that's the case.
“At this time there is no indication of criminal activity,” a CTA official told NBC 5 by phone.
Sources within the CTA told NBC 5 that investigators are pursuing the idea the train was stolen from the Forest Park train yard known as "The Jungle."
CTA spokesman Brian Steele said there was no sign of criminal activity in or around the car.
"There's no broken windows, there's no pried-open doors, there's no graffiti or vandalism inside the rail car," Steele said, "but we are doing a thorough investigation of this."
Amalgamated Transit Union president Robert Kelly said during a news conference it's still unclear what happened and how the train got out of the station.
"Both the supervisor in the station at Forest Park and the motorman who was sitting in the station said there was nobody on the train as it went through and collided with the other train," Kelly said.
He said the train included cars that have been on hold since Sept. 23 awaiting repair at the Skokie Train Yard. He said it takes two keys to start -- one to get in the motorman's booth and one to turn on the train. He said the second key is universal.
The Forest Park yard is elevated from the rest of the tracks so the train involved had to have power applied to it to get into the yard. It somehow got through three interlocks and overcame the "dead man's switch" to continue.
"This is baffling everybody," he said.
Under CTA procedures, two trains are not supposed to be in the same spot and there is fail safe equipment in place to help make sure that crashes like this do not happen.
The National Transportation Safety Board is sending investigators to Chicago to investigate today's collision of a CTA commuter train.
Sources, who caution that this is only a working theory, said whoever was driving the train was wearing a CTA uniform and possessed the universal key required to start the train and had knowledge of the train's override systems.
A witness on the scene, Martinez Butler, a locomotive engineer for another train company, was standing at the corner when the trains collided.
"There was a train that was stopped at the station. Another train came through and looked like it blew the signal, because I heard beeping," she said. "I'm a locomotive engineer. I know the systems, I know the sounds. When you hear those beepings it's warning you that there is an obstruction in front of you and you need to stop."