LEWISTON — An off-duty police officer is credited with helping to clear a pair of Webster Street homes after one went up in flames early Monday morning.

After quickly evacuating his own nearby house, Officer Nick Meserve alerted tenants of the burning building at 97 Webster St. that their home was on fire.

Authorities on Tuesday said the blaze that severely damaged the two-apartment house was arson.

Sgt. Joel Davis at the Office of the Maine State Fire Marshal said the fire started in the area of the building’s front porch by an “intentional human element.” Lewiston Fire Inspector Paul Ouellette said the fire started at about 2:30 a.m. at the front of the building, near a 120-gallon propane tank. A relief valve on the tank that was expelling gas was shooting flames like a blowtorch.

Meserve was at his nearby Webster Street home watching “Game of Thrones” at about 2 a.m. Monday when he heard a strange sound from outside. After checking to make sure nothing was afoot, he went to bed.

He didn’t stay there for long.

“I woke up to a muffled popping,” Meserve said Tuesday. “I went and investigated the source of the sound. I walk into my kitchen to see the front porch of the neighbors’ house fully engulfed and propane shooting a 15- to 20-foot flame up the side of the porch.”

The officer cleared his own building and then hollered to the tenants of the burning house to alert them to the blaze. Minutes later, all tenants had escaped to safety and firefighters were at the scene.

Fire crews battled the blaze for two hours before it was brought under control.

Ouellette said the building, which was insured, sustained about $130,000 in damage. The two tenants, who had insurance, were assisted by the American Red Cross of Maine.