The surprise shutdown of the Hood Canal Bridge on Sunday was caused by human error, officials with the state Department of Transportation said Monday.

The Highway 104 span over the Hood Canal was closed for nearly seven hours Sunday, July 10 to both vehicle and marine traffic. Officials said the lift span was not even when it opened, and was caused by a malfunction of one of four hydraulic lift cylinders that raise and lower that section of the bridge.

Officials said the lift didn’t have enough hydraulic fluid to work properly because a valve had been inadvertently left closed following preventive maintenance activities earlier in the week.

The malfunctioning hydraulic lift caused the lift span to rise unevenly during an 8 a.m. opening Sunday for marine traffic.

“We are so sorry for the inconvenience and travel disruptions this incident caused to mariners and motorists alike,” said Troy Cowan, the Olympic Region assistant regional administrator of maintenance and operations for the state transportation department.

“Our crews take great pride in their maintenance of the Hood Canal Bridge. This mistake is a humbling reminder of how important their work is, and how important it is for them to be focused and diligent at all times,” he said.

When the incident happened on Sunday, bridge engineers and additional bridge crew were sent to the bridge to manually lower the span to its closed position. The bridge was then inspected for damage before reopening to traffic.

Initial assessments revealed only minor damage; a few broken bolts used to mount an electrical limit switch, which was also broken.

As a result of Sunday’s shutdown, officials said policies for opening the lift span will be revised to include more quality assurance checks and additional test openings of the span following specific maintenance procedures.

“We are taking these steps to ensure that this type of event does not occur again, as we understand how vital this bridge is to Olympic Peninsula residents and businesses,” Cowan said.

There are two maintenance openings scheduled for 12:30 and 2:30 p.m. Monday, July 11, at the bridge.

The maintenance openings were scheduled prior to Sunday’s unexpected closure.