Arson was the cause of a fire last spring on a 140-foot boat unlawfully moored off Whidbey Island, according to a report by the King County Sheriff's Office.

It’s official: Arson was the cause of a fire last spring on a 140-foot boat unlawfully moored off Whidbey Island, according to a report by the King County Sheriff’s Office.

The Deep Sea, an old, rusted-out fishing vessel, caught fire and sank to the bottom of Penn Cove early one Saturday in May. It cost state and federal government nearly $5.4 million to clean up the mess and dispose of the craft.

The state Department of Health also shut down Penn Cove shellfish harvesting for over two weeks after thousands of gallons of diesel fuel spilled into an area internationally renowned for mussels.

Sheriff’s investigators recently determined the fire was intentionally set, but there are no suspects, according to Toni Droscher, a spokeswoman for the state Department of Natural Resources (DNR).

The boat had been left in the cove at the end of December 2011 by its owner, a Maple Valley scrap dealer named Rory Westmoreland. The state had begun assessing fines for the illegal moorage, and considered towing it away before it sank.

Droscher said DNR sent Westmoreland a $1.3 million bill in an attempt to recoup its expenses in the cleanup. However, the agency conceded that it’s unlikely to collect. It is also looking at possible criminal charges related to leaving a vessel derelict and abandoned, Droscher said.

Westmoreland had bought the boat just six months earlier for $2,500, after the Port of Seattle advertised it on Craigslist. He had planned to use it to retrieve garbage and abandoned nets from the sea, but the idea never got beyond the talking stage.

Material from Seattle Times archives

included in this report.

Maureen O’Hagan: 206-464-2562 or mohagan@seattletimes.com