The fire was reported just before 4 a.m.
Two yachts were destroyed, and two people in another boat had to be rescued by firefighters. Flames shot 50 feet into the air.
Snohomish County Fire District 1 investigators ruled the fire "undetermined," meaning there wasn't enough information available to pinpoint an exact cause, district spokeswoman Leslie Hynes said. There was some evidence indicating an electrical problem.
"It was undetermined, but there were concerns about the electricity," she said.
They found nothing to indicate the fire was suspicious.
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