Mistakes made by Washington State Ferries personnel were to blame for the failure of an electric drive motor that knocked the 188-car ferry M/V Walla Walla out of service.

WSF said a three-month safety investigation revealed that “human error” was the cause of the motor failure.

The motor failure happened Nov. 4 when the Walla Walla was undergoing routine annual maintenance at Eagle Harbor Maintenance Facility on Bainbridge Island.

Investigators said the failure was avoidable.

The mistakes of WSF workers included:

Communication between engine crew and maintenance staff was insufficient;

Eagle Harbor maintenance staff and the Walla Walla’s engine crew improperly prepared electrical equipment for a complex maintenance procedure; and

No written procedures or checklists were available for this maintenance work, which is rarely performed on the Jumbo-class Walla Walla.