An electrical fire that spread to a 15-foot Christmas tree prompted a blaze that reduced a 16,000-square-foot riverfront mansion near Maryland's capital to ruins, killing a couple and four of their young grandchildren, investigators said Wednesday.

The fire ignited combustible material, probably a tree skirt, and tore through the massive, castle-like structure in the early morning hours of Jan. 19.

Anne Arundel County Fire Chief Allan Graves said the tree had been cut more than 60 days before the blaze and was in a "great room" of the house with 19-foot ceilings.

"The involvement of the Christmas tree explains the heavy fire conditions found by the first arriving fire crews," Graves said.

Investigators on Wednesday identified the victims as Don and Sandra Pyle and their grandchildren: Charlotte Boone, 8; Wes Boone, 6; Lexi Boone, 8, and Katie Boone, 7. Don Pyle, 56, was chief operating officer of ScienceLogic in Reston, Virginia.

The fire was reported about 3:30 a.m. Jan. 19 by an alarm-monitoring company, reporting smoke had been detected inside, and a neighbor who spotted flames. The home had smoke detectors, and there was no indication they did not work, said Deputy Chief Scott Hoglander of the Anne Arundel County fire marshal's office.

The big tree fueled the fire, which spread rapidly. The 911 call from a neighbor came within 2 minutes of the report to the alarm-monitoring company.

"I think it's more about the actual fuel load of the Christmas tree and the output of energy and heat from that particular fuel load that caused the rapid fire spread," Hoglander said. "It really had nothing to do with the building construction itself."

The investigation found that a failure in an electrical outlet in the floor that provided power to the tree produced heat that ignited something combustible, probably a tree skirt, said Russ Davies, a spokesman for the Anne Arundel County Fire Department.

Some 85 firefighters from several jurisdictions fought the four-alarm fire, which burned for three hours before it could be contained. Because there was no hydrant in the area, firefighters shuttled tankers to the site and stationed a fire boat at a pier nearby.

Investigators brought in dogs to search for bodies and evidence, such as accelerants, and conducted more than 50 interviews. Bill McMullan, special agent in charge of the Baltimore field office of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, said the investigation concluded the fire was the result of "a tragic accident that occurred at the absolutely worst possible time, while the Pyles and their grandchildren were sleeping."

Hoglander declined to mention a specific cause of death, because officials have not received an official report back from the state medical examiner's office.

A spokeswoman for the children's parents said that the day before the fire, the doting grandparents bought the children costumes before taking them to dinner at a medieval-themed restaurant.

Charlotte and Wes Boone were sister and brother. Lexi and Katie were sisters; they had a newborn brother who was home with his parents, Randy and Stacey Boone, the night of the fire. The cousins' fathers, Randy and Clint Boone, were the sons of Sandra Pyle, 63. The four children were students at the Severn School in Severna Park.

In a statement issued Wednesday, the Boone family thanked investigators for their work and well-wishers for their prayers.

"While the explanation that has been shared with us today does not bring solace, it does start us down the long road to acceptance," the statement read.

The Pyles built the home in 2005, four years before the county began requiring sprinkler systems in new homes. Hoglander said he believes sprinklers would have made a difference.

"I would say without a doubt," he said.

The $6 million property once boasted turrets, spiral staircases, lion statues, a sprawling lawn and forested land. All that remains resembles a colonial ruin: a brick wall with windows missing and a mountain of burned debris.

As investigators from the fire department, the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, and the state fire marshal's office probed the scene, members of the community brought notes and teddy bears for a small memorial just outside the property. On brick columns that flanked an iron gate, Christmas decorations were still displayed.