WATERTOWN, S.D. (AP) — Investigators have concluded that frayed wires on a heating pad sparked a fire that destroyed a 41-unit senior housing complex in Watertown.

Fire Rescue Chief Doug Kranz tells KXLG radio that officials with the state fire marshal's office and the federal Bureau of Alcohol, Tobacco and Firearms helped determine the cause of the blaze late Tuesday and early Wednesday at the Sunchase Apartments.

Kranz says the fire went from the heating pad up the wall and into the attic, where it quickly spread throughout the complex.

No serious injuries were reported but all 43 residents have been displaced.

Copyright 2013 The Associated Press.