The cause of a fire that destroyed a third-floor apartment on Grant Street today remains under investigation.

Deputy Fire Chief Bill Flynn said a motorist called 911 at 12:54 p.m. to report heavy smoke and flames shooting out of the top floor of the apartment building at 33 Grant St. It took firefighters about 30 minutes to knock down the fire, which was contained to the third floor.

Flynn said there were furnishings in the apartment but it was unclear whether anyone lived there. The first- and second-floor apartments were not occupied. Those units sustained water damage.

Flynn said tonight that city officials were still trying to find the building’s owner.