BOSTON - A six-car train carrying about 50 passengers left a suburban Boston transit station without a driver Thursday and went through four stations without stopping, and investigators were focusing on "operator error" as the reason why.

The trip began shortly after 6 a.m. at the Braintree station and ended about nine minutes later when power was cut to the rails, transportation officials said. No passengers were hurt.

Stephanie Pollack, the state's transportation secretary, said the investigation was focusing on the Massachusetts Bay Transportation Authority train's operator, who had sought and received permission to move the train despite a signal problem.

The operator, a 51-year-old man with more than 25 years of experience with the MBTA, got off the train to execute a procedure to put the train into "bypass mode." The probe, Pollack said, was focusing on whether the operator failed to properly secure the braking system. The employee suffered a minor injury when he was brushed by the train as it moved away from the station, officials said.

The incident was an "unacceptable breach of our responsibility to keep our riders safe," Pollack said at a late afternoon news conference.

Gov. Charlie Baker said it appeared the train had been tampered with, although he also noted investigators had not determined whether the tampering was due to negligence or something else, and called the incident "isolated."