Officials said that while the incident was likely mechanical failure, they were considering the possibility that it was "deliberately set in motion."
"We have no indication at this point that there has been any criminal activity but we are doing a thorough investigation," said Chicago Transit Authority spokesman Brian Steele. "Everything that is a possibility is being looked at."
Steele said the empty train was moving at an estimated 20 miles per hour when it slammed into the train waiting at the station. Fail-safes intended to prevent a runaway train did not properly function.
Amalgamated Transit Union president Robert Kelly said the train had been at a repair yard awaiting a move to Skokie for maintenance, but they would not have done so during rush hour.
"Mechanical malfunction is easy to say but hard to explain," Kelly said. "For it to move this morning, someone possibly keyed up the train. I have never seen a train just start up and start moving -- ever."
A CTA spokeswoman said 33 people were taken to hospitals with minor injuries. Service along the Blue Line was temporarily suspended during rush hour.