SEATTLE -- Authorities are working to determine what caused an oil spill that appeared Thursday on Lake Union.
An oil sheen was spotted covering about two acres of the surface of the water near the lake's east shore.
Forty gallons of the substance were recovered from the water. Crews deployed a boom to contain the spill and set up cleanup materials along piers and the shoreline to trap more oil overnight.
The Department of Ecology has received no reports of harm to wildlife.
Department of Ecology officials said the substance has a petroleum odor, and a sample was collected for testing.
A partially sumberged houseboat is floating nearby, but investigators said they are not sure whether it's responsible for the spill.