SOUTH PORTLAND – Investigators theorize a natural gas explosion that leveled a vacant house took place when a pilot light in a water heater ignited gas that could have entered the two-story building through an empty sewer or water pipe.

The blast has been declared an accident, but State Fire Marshal Sgt. Joel Davis said the precise chain of events remains uncertain.

No one was injured in the Monday morning explosion on D Street in the Knightville neighborhood, near the Mill Creek Shopping Center. The blast took place about an hour after a backhoe being used by a construction worker to replace a water main clipped a gas line.

Much of the neighborhood was evacuated, and residents closest to the blast scene weren’t able to return to their homes until the next day.

Workers have replaced the natural gas main on D Street and service has been restored throughout the area.

The State Fire Marshal’s Office has closed the investigation because there is no criminal aspect to the case, but an investigator for the Maine Public Utilities Commission is trying to figure out how the gas got from the break in the line to the house.

Another PUC investigator is looking into whether the utility and the excavator followed safe digging rules.