UPDATED: 10/1/2013 5:30 a.m.
CHICAGO (CBS) -- Authorities are trying to determine what caused two CTA Blue Line trains -- one of them apparently without an operator -- to collide Monday morning and injure dozens of rush-hour passengers.
A westbound train with about 40 passengers was standing on the tracks near Harlem Avenue in Forest Park when an eastbound train -- without a controller -- traveling on the same track smashed into it.
Police were investigating the possibility that somebody deliberately caused the accident. However, CTA and federal officials believe the crash was likely an accident. The National Transportation Safety Board was in charge of the investigation.
On Tuesday morning, the trains remained at the Harlem station, which is closed for the morning rush hour. The Blue Line will stop at all other stations, while the NTSB continues its investigation.
Suspicion that the train was stolen or hijacked was raised because the train that caused the crash only had four cars. Rush hour trains typically have many more cars and would not have left the train yard with so few cars.
The CTA has confirmed that the eastbound train was on the wrong tracks.
Forest Park Mayor Anthony Calderone told CBS 2's Mike Puccinelli that nobody was at the controls of the train when it crashed around 8 a.m.
Police are treating the accident as a crime scene. Later Monday, white canvas panels concealed the spot where the two trains had collided.
Brian Steele with CTA says there is no indication of criminal activity. The CTA is working under the theory that the accident was due some sort of mechanical malfunction.
The train that hit the standing train had been marked "out of service," said Steele, who said the collision occurred at a "pretty slow speed."
Federal officials say on background that all indications are that the crash was an accident.
Robert Kelly, of Amalgamated Transit Union 308, said somebody would need a key to fire up and then drive the train from the Forest Park train yard, where one end of the Blue Line originates.
"The million dollar question is, how did this happen?" Kelly said.
Kelly said surveillance cameras that could have recorded the rogue train's departure were not working.
At least 33 people were hurt when the trains collided in west suburban Forest park, according to the CTA. All of those injuries were on the standing westbound train. There were a total of about 40 people on that train.
"We were standing still and then all of a sudden something hit us and I got shot forward into the front seat," said one passenger as she limped to a responding ambulance.
Kelly told CBS 2 the train may have been traveling upwards of 20 to 25 miles per hour when it hit the stationary train, and its unclear if there was a motorman on the moving train when the collision happened.
None of the injuries was believed to be life-threatening.
One of the many questions: How did the eastbound train manage to leave the Forest Park rail yard and pass over at least two interlocking systems that should have triggered the brakes?
In addition, a red signal alerting a driver should have gone off, forcing the driver to apply the brakes.
Preliminary information indicates some of the cars on that eastbound train were undergoing maintenance in the rail yard, sources said. How the trains managed to leave the yard and travel past the Forest Park terminal and down the tracks to the next stop at Harlem remained unknown.
The conductor of the westbound train saw the approaching train and did what he could to alert passengers to take over or brace themselves for impact.
That conductor walked away from the accident, but did have some facial injuries.
Service was suspended between Forest Park and Austin Avenue, along the border of Chicago and Oak Park. Service was eventually resumed to Forest Park, but trains were not stopping at Harlem.
Images from Chopper 2 HD showed people being taken to ambulances on stretchers.
The CTA was providing shuttle buses to transfer affected Blue Line riders to the Green Line.
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