The State Fire Marshal’s Office is investigating an explosion and fire that destroyed a house in Union early Thursday morning.

The two-story house on Sunk Haze Lane in the Knox County town of Union was leveled by the explosion around 3 a.m. All occupants of the house have been accounted for.

Investigators say the house was under construction and the occupants were staying elsewhere. The house was being heated with portable heaters, said Stephen McCausland, spokesman for the Maine Department of Public Safety.

The house is in the center of town.

A team of fire marshal investigators has been on the scene for hours and began looking through the rubble at daylight, McCausland said.