Boston Runaway Train Operator May Have Tied Cord to Throttle, Source Says

— -- Early findings indicate that the operator of the runaway Boston train that left a station and stormed through four stops without a conductor tied a cord to the throttle and did not set at least one of the brakes before exiting the train, a source familiar with the investigation told ABC News today.

The six-car train on Boston's Red Line left the Braintree Station without an operator just after 6 a.m. Thursday, according to Stephanie Pollack of the Massachusetts Department of Transportation.

It traveled several stops northbound toward Boston without an operator 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.

"The investigation is ongoing but we do believe this was an isolated incident," Massachusetts Gov. Charlie Baker said today during a news conference. "One that likely required an individual to make multiple mistakes for this to take place."

Passengers spent 9 minutes on board the driverless train without any communication with transit officials.

While other passengers may be nervous about using the trains, said Baker, "We are confident that this was an isolated incident where a single individual appears to have made multiple errors."

"Those errors led to a train leaving the station without an operator on it. But within a very short period of time," he said, the problem was quickly identified and officials began the process of shutting down the rail and moving other trains out of the way.

This singular incident doesn't justify adding a second operator, Baker said.

The operator remains on administrative leave, said Pollack, adding that, “If the investigation finds that a prohibited act occurred, this is grounds for termination.”

The train itself is also being investigated to determine if everything was functioning properly, officials said today.

"We have no reason to believe it is a widespread practice," Pollack said, but the department is taking this opportunity to investigate.

There are no cameras in the control cars, Pollack said.

"We are reminding our work force ... that they are not permitted to in any way keep that throttle ... from operating the way it is intended," Pollack added.

Baker said on Thursday 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.

The operator of the train exited the cab after a "signal issue," Pollack said, noting that normally two different types of brakes are supposed to be used during such an event, including a manual break.

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, Michele McPhee and the Associated Press contributed to this report.