Fire investigators are looking into the cause of a two-alarm blaze that caused heavy damage to an Old Main Street home early this morning.

There was heavy damage to the rear of the building at 274 Old Main St., though the front portion of the building appeared to be virtually unscathed. The heaviest damage appeared to be in the crawl space under the flooring.

According to an insurance adjuster on the scene, homeowners Paul and Lisa Pereira and their family were away for the weekend and the home was vacant at the time of the fire.

A 911 call from a neighbor at 5:17 a.m. alerted the Fire Department to flames and smoke coming from the home. According to Firefighter Jim Bruce, crews from all three Tewksbury stations responded to the scene, with station coverage provided by units from Billerica, Andover and Wilmington. Billerica also sent a truck to the fire itself, for a short time.

There was a vehicle belonging to the homeowners in the driveway of the residence, leading firefighters to initially believe people might be inside, according to Bruce. This resulted in an interior search of the building, while firefighters were also attempting to extinguish the flames.

Firefighters were on the scene for about two hours.

Lt. Scott Keddie is leading the investigation into the cause.