33 people were reportedly injured on Monday morning after two trains collided with one another, as an empty one may have been 'deliberately set in motion,' ABC News reported. Authorities said all injuries are believed to be minor but the cause of the accident remains highly suspicious.

A law enforcement official said that an empty train, which had been was out of service rolled east on an "El" track, slamming into the west bound train, which had been loading passengers on the CTA service close to 8 a.m. on Monday morning.

While terrorism was ruled out, authorities continue to investigate whether the westbound train had been "deliberately set in motion," news reports said.

"We have no indication... that there has been any criminal activity, but we are doing a thorough investigation. Everything that is a possibility is being looked at," said Ryan Steele, a CTA spokesman.

Anthony Calderone, the Forest Park Mayor, said that "more and more it seems to be leaning towards a mechanical failure."

But the president of the Almagamated Transit Union Robert Kelly disagreed: "Mechanical malfunction... is easy to say but hard to explain. 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," he said.

Service along Chicago's blue line was temporarily suspended on Monday morning after the crash. Service resumed a few hours later, as Catherine Hosinski, a CTA spokeswoman, said an investigation is underway to determine the cause of the accident, ABC News reported.