FRANKLIN, Mass. — A propane gas leak was the cause of an explosion at a condominium complex in Franklin that claimed the lives of a married couple last fall.

State Fire Marshal Peter Ostroskey said in a statement Wednesday that Nancy and Richard Brown, both 66, were killed in the October 2015 blast.

Investigators say nothing indicating that any criminal or intentional act sparked the explosion was found.

Fire officials say the explosion occurred when an unknown source ignited gas that had leaked into the couple’s home. The propane may have possibly come from the Browns’ heating system or fireplace.

The couple had recently moved from Florida to Massachusetts to be closer to their grandchildren.