BOSTON (AP) — A six-car train with passengers on board that left a suburban Boston transit station without a driver Thursday and went through four stations without stopping was tampered with, Massachusetts Gov. Charlie Baker said.

None of the approximately 50 passengers was hurt, but the train's operator suffered a minor injury when he was brushed by the train, apparently as it began to move at the Braintree station, a spokesman for the Massachusetts Bay Transportation Authority said.

The above-ground Red Line train departed Braintree Station — the southernmost stop of the line — shortly after 6 a.m. without an operator and traveled north toward Boston, a statement from the MBTA said.

MBTA operations eventually disabled the train and brought it to a stop by cutting off power to the electrified third rail, officials said. An initial investigation indicated that a safety device within the train's cab may have been tampered with.

"This train was tampered with, and it was tampered with by somebody who knew what they were doing," Baker said during an interview on Boston Herald Radio.

Baker called it an "isolated" incident and said MBTA passengers should not be concerned.

Transit personnel boarded the train after it was stopped and drove it north to the JFK/UMass stop, where passengers disembarked. The train was taken out of service and brought to a maintenance facility in Boston, where an investigation is under way, according to Joe Pesaturo, spokesman for the transit agency.

Passengers are among those being interviewed, the T said.

Kristen Setera, a spokeswoman for the Boston office of the FBI, said in an email that the agency was aware of the incident and was in contact with transit police, but provided no other information.

Pesaturo said an initial examination showed no problems with the "functionality" of the train's equipment.