WATERFORD — A Thursday morning fire destroyed the Mallard Mart on Valley Road, taking firefighters from six towns about three hours to extinguish it.

The cause is under investigation. There were no injuries, officials said.

The fire was reported at about 10 a.m. at the convenience store and gas station at 972 Valley Road. Assistant Chief Tom Murch of the Waterford Fire Department said the fire was well under way with the inside walls aflame when firefighters arrived.

Thick white smoke billowed for hours from the building, which sits at the intersection of Routes 35 and 118 in North Waterford. Murch said there was an apartment in the building with a single resident, who was at work when the fire broke out. He lost everything, Murch said.

The building was insured.

The owner, Ed Michaud of Norway, was in the building when the fire broke out, Murch said. He said the smoke detectors went off Thursday morning, and smoke was found coming out of a back room at the store, prompting a customer to try to put out the fire with a fire extinguisher.

The cause is under investigation, Murch said. An investigator from the state Fire Marshal’s Office was on the scene Thursday and will be back Friday.

Murch said the fire is believed to have started in the store. The store also sells gasoline, but Murch said the gas tanks were never in danger from the flames. “That was the only gas in town,” Murch said.

The building was about 40 by 60 feet and was two stories high, Murch said. It was once owned by Ed Packard who ran an auto repair business there in the 1950s and ’60s.

Traffic on Route 35 between Waterford and Stoneham was blocked for about three hours. Assisting Waterford firefighters were departments from Stoneham, Harrison, Oxford, Norway and Paris. Paris and Oxford supplied ladder trucks, and firefighters used them to douse the flames from above.