WILLMAR - Investigators are working to determine the cause of a fire Monday evening at a vacant building on the MinnWest Technology Campus in Willmar.

Willmar Fire Chief Gary Hendrickson said the blaze was reported just after 9 p.m. Monday when it was spotted by a Willmar police officer.

By the time the first Willmar fire engine arrived, heavy fire was coming through the roof of the building, which was vacant but was undergoing renovations.

The concrete construction of the 1930s vintage building stopped the fire from burning the structure, but it essentially burned the roof off, Hendrickson said.

No one was injured.