ANNAPOLIS, Maryland — An electrical failure ignited a large, dry Christmas tree and sparked a fire that tore through a Maryland waterfront mansion last week, killing six family members, investigators said on Wednesday.

The Annapolis blaze on Jan. 19 was ruled accidental, Anne Arundel Fire Department Chief Allan Graves said at a news conference.

Killed in the fire were Alexis Boone, 8, her sister Kaitlyn Boone, 7, their cousins Wesley Boone, 6, and Charlotte Boone, 8, and their grandparents Don Pyle, 56, and Sandra Pyle, 53.

Don Pyle was chief operating officer of the privately held technology company ScienceLogic.

Investigators found that a failure in a nearby wall outlet spread to the 15-foot tree, which stood in an atrium of the 16,000-foot home, Graves said.

Sleeping and living areas were connected to the room, Graves said, adding that the fire quickly spread and engulfed the home, located about 30 miles east of Washington, D.C.

Workers needed several days to recover the six bodies from the rubble, after the home’s first, second and third floors had collapsed into the basement.