CHICAGO, Ill. -- An empty train possibly "deliberately set in motion" slammed into a Chicago commuter train this morning, injuring dozens and bringing the rush hour to a halt.
Authorities said 33 people were taken to hospitals, but their injuries are believed to be minor, a Chicago Transit Authority spokeswoman told ABC News.
A law enforcement source familiar with the investigation said an empty train that was out of service rolled east on an "El" track and slammed into a westbound train that was loading passengers on the CTA's Harlem stop. The crash occurred at 7:47 a.m.
The source said authorities are looking into whether the train area was "deliberately set in motion." The source says there is no information to indicate terrorism.
Police in Forest Park, Ill., the western suburb where the Harlem stop is located, told WLS that the crash was being treated as a crime scene.
"We have no indication at this point that there has been any criminal activity, but we are doing a thorough investigation," CTA spokesman Ryan Steele said. "Everything that is a possibility is being looked at."
Forest Park Mayor Anthony Calderone said the accident was likely due to "mechanical failure."
"More and more it seems to be leaning towards a mechanical failure," Calderone said at a press conference this morning.
But Robert Kelly, the president of the Amalgamated Transit Union, noted that a "mechanical malfunction… is easy to say but hard to explain."
"For it to move this morning, someone possibly keyed up the train," he said. "I have never seen a train just start up and start moving – ever."
The collision was a harrowing jolt for people at the Harlem stop.
"I was waiting for my train. Time to go to school. I heard screaming, 'Stop the train, stop the train, slow down,'" Taylor Pettigrew, who witnessed the crash, told ABC's Chicago station WLS.
In the wake of the collision, service along the blue line was temporarily suspended. Around two hours later, service resumed with trains using a single track, but without stops at the Harlem station. Traffic ground to a halt along I-290, the Eisenhower Expressway.
CTA spokeswoman Catherine Hosinski said an investigation is underway to determine the cause of the accident.