An empty Chicago commuter rolls down the track and slams into a suburban line, injuring dozens at morning rush hour Monday.
Bringing a bustling commute to a halt, the mystery has investigators now scrambling for clues.
“The head-on collision came during the morning commute, and sent 33 people to the hospital,” reports CNN's Ted Rowlands. “Surveillance video, and witness accounts indicate that no one was at the controls of the runaway train, which was traveling at an estimated 20 miles an hour at impact.”
Investigators initially suspected that someone intentionally sent an empty four-car train-head on into an eight-car-train full of commuters, but at this point, they haven't found any evidence of wrong doing.
“There’s no broken windows, there’s no pried open doors, there’s no graffiti or vandalism inside the rail car,” says Brian Steele of Chicago Transit Authority. “But we are doing a thorough investigation of this.”
However, transit officials say someone must have started the train, which had been sidelined for repairs.
According to Union President Robert Kelly, “It should have gone through a series of interlocks and the train should have been tripped and went into what we call emergency."
The question now is why and how come these two separate systems failed.
NTSB Investigator Tim Depaepe says the agency is exploring all the possibilities. “If it’s not signal, it’s not mechanical, then we look at operations, because then it might be a human factor accident."
Follow along at CNN.com for developments.