-- Officials are now investigating "operator error" after a train on Boston's Red Line traveled several stops without an operator this morning, according to Stephanie Pollack of the Massachusetts Department of Transportation.
The six-car train left the Braintree Station without an operator just after 6 a.m. and traveled northbound toward Boston until it was finally brought to a stop when officials cut off power to the third rail. None of the approximately 50 passengers were hurt.
Massachusetts Gov. Charlie Baker said earlier that a safety device within the train's cab may have been tampered with. Pollack said operator error is likely the cause of the incident.
According to Pollack, the operator of the train exited the cab after a "signal issue." She said normally two different types of brakes are supposed to be used during such an event, including a manual break.
Passengers spent 9 minutes on board the driver-less train without any communication with transit officials. The operator has been put on administrative leave. The train has been impounded and is no longer in service.
Pollack said the incident was an "unacceptable breach of responsibility to keep our riders safe. We failed our passengers. Something happened that should not have been able to happen.”
The National Transportation Safety Board has been made aware of the incident but is not contributing to the investigation.
ABC News' Aaron Katersky and the Associated Press contributed to this report.