The FBI is investigating possible sabotage after a dozen railroad spikes were reported missing from a section of commuter train tracks on Chicago's South Side.
The spikes hold down metal plates that bind the rails to wooden ties underneath.
"If a sufficient number of spikes are removed in a contained location, there's the potential for the rail to shift, which would lead to disastrous results and train derailment," said Federal Railroad Administration spokesman Steve Kulm.
Metra workers discovered the missing spikes on Monday and notified police and federal authorities, including the Department of Homeland Security and the Transportation Security Administration, which investigates threats to planes and trains.
Checking ties to domestic violence case
The FBI said it was conducting a criminal investigation into "sabotage." FBI spokesman Ross Rice said agents also were checking for possible connections to a domestic violence case involving a Metra engineer.
"No one has claimed responsibility or called in a threat," Rice said. "No similar incident has been reported to us in the metropolitan area. This is a serious incident, and we are aggressively investigating."
Gunshots were fired at two Metra trains on the same line in June, about two miles south of where the spikes were removed.
Metra officials were concerned the target of the shots may have been an engineer who is allegedly a victim in a domestic violence case, said agency spokeswoman Judy Pardonnet.
The affected tracks carry three commuter lines, including the Metra Electric Line to University Park and Blue Island, with around 40,000 riders daily, and the South Shore Line to Michigan City and South Bend, Ind.