Mystery of Chicago's Runaway Train Crash
PHOTO: CTA trains collided in Chicago

An empty train rolled down a Chicago el track and slammed into a commuter train that was loading passengers this morning, injuring dozens and bringing the rush hour to a halt.

Officials investigating how the train got rolling are leaning towards a mechanical failure, but they are also trying to determine whether it was "deliberately set in motion," a law enforcement source familiar with the investigation told ABC News.

"We have no indication at this point that there has been any criminal activity… but we are doing a thorough investigation. Everything that is a possibility is being looked at,." Chicago Transit Authority spokesman Brian Steele said.

Tim DePaepe, an investigator for the National Transportation Safety Board, held a news conference this afternoon to say they have "no probably cause at this time."

Authorities said 33 people were taken to hospitals, but their injuries are believed to be minor, a CTA spokeswoman said.

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 train was estimated to be traveling at 20 mph when it hit, Steele said.

The source said authorities are looking into whether the train area was "deliberately set in motion."

The train's entire trip may have been captured by surveillance cameras, officials said. Fail-safes intended to stop a runaway train did not function properly. officials said.

The empty train had been at a repair yard since Sept. 23 awaiting a move to Skokie for maintenance. Robert Kelly, the president of the Amalgamated Transit Union, said they would not have moved that train to Skokie in the middle of rush hour.

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 Kelly 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.

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