State officials have determined that it was operator error that lead to yesterday's incident where a Red Line train that left Braintree Station a driver.

The MBTA's investigation is centered on the actions of the operator, who Gov. Charlie Baker says would have had to make big mistakes in order for the train to run by itself.

"We are confident that this was an isolated incident where a single individual appears to have made multiple errors," Baker said.

Baker said previously that the controls had been tampered with, but because of the ongoing investigation, he wouldn't confirm reports that the operator jury-rigged the train's controls.

When the operator exited the cab to switch the train to manual mode, it took off at an estimated 25 miles per hour, carrying about 50 passengers through four more stations before power to the third rail was stopped.

The MBTA will now require supervision whenever a train is switched to the manual mode.

The 28-year veteran operator is on paid administrative leave, but could be fired if it's determined that he did in fact rig the accelerator.