Authorities are investigating a fire that burned 4 acres on Sheep Island in Casco Bay last week, saying the cause is suspicious.

Department of Agriculture, Conservation and Forestry spokesman John Bott confirmed in an email Tuesday night that the fire was considered suspicious, but did not offer any specifics other than to say it was under investigation.

Maine firefighters and forest rangers saved 10 cottages on the Harpswell island from the fire, which was reported around 2 p.m. Friday.

Two people were on the 9.4-acre island during the fire, which spread quickly because of dry conditions and a large number of dead fir trees.