Police and firefighters are trying to figure out how a pair of boats caught fire and sank in Cowichan Bay over the weekend.

Witnesses reported seeing flames break out on the unoccupied boats, which were moored together, at around 7 p.m. Saturday night.

Volunteer firefighters launched a boat to battle the fires, but both vessels quickly burned and sank into the bay.

A third boat broke away from the other two as a result of the fire and was later towed to shore.

The Coast Guard also launched two vessels in response but it is unclear whether any fuel leaked into the water.

The owner of all three of the vessels is cooperating with police, RCMP said.

Investigators are still trying to pinpoint a cause, and say the fires appear to be suspicious.

Anyone with information is asked to call North Cowichan-Duncan RCMP at 250-748-5522 or Crime Stoppers at 1-800-222-8477.