State investigators have not been able to determine the cause of a fire that destroyed a building supply store in Winslow on Friday.

The fire started around noon near a shed in the back of McCormack Building Supply that stored white pine lumber, according to Stephen McCausland, spokesman for the Maine Department of Public Safety. It then spread, destroying the main building and causing more than $4 million worth of damage.

After interviewing staff and customers, along with workers from a paving company that was working on the property, investigators from the Maine State Fire Marshal’s office have ruled the cause of the fire “undetermined,” according to McCausland. He said the extensive damage made it impossible to conclusively determine the source.

Owners of the business intend to rebuild the store and lumberyard, according to the Morning Sentinel.