Jan. 19, 2015: In this file photo, firefighters battle a four-alarm fire at a home in Annapolis, Md. (AP Photo/Capital Gazette, Glenn A. Miller, File)

ANNAPOLIS, Md. –  An exhaustive six-month probe has determined a damaged electric plug under a large Christmas tree sparked a mansion fire that spread so quickly a Maryland couple and four of their grandchildren couldn't escape, despite a functioning alarm system, according to report released by federal investigators.

The Bureau of Alcohol, Tobacco, Firearms and Explosives and the Anne Arundel County Fire Department report, which concluded the fire was accidental, identified numerous hazards at the castle-like mansion near Annapolis, Maryland. The tree had shown signs of moisture loss at the time of the fire, and it was scheduled to be removed the day after the blaze, the report said.

"When this tree caught on fire, within seconds that entire room — instead of being a fire in a room — turned into a room on fire, meaning that all of the combustible gases and all the fuel packages in that room were instantaneously ignited," said Special Agent David Cheplak, of the ATF's Baltimore field office.

The report noted that although the home was a residential building, it was built like a commercial building with about 16,000 square feet, not including the basement, the report said.

"Additionally, the building had been built to look like a castle so there were numerous round turrets located throughout the building," the report said. "All of these factors created potential collapse issues."

Investigators determined a 15-foot Fraser fir Christmas tree in the mansion's Great Room was capable of generating enough energy to fuel the blaze. They also found there were no timers on the Christmas lights, which had been left on for about six weeks leading up to the fire. The tree also had shown signs of moisture loss at the time of the Jan. 19 fire at about 3:28 a.m. that killed Donald and Sandra Pyle and four of their grandchildren, Charlotte Boone, 8; Wes Boone, 6; Lexi Boone, 8, and Katie Boone, 7.

Other potential causes were identified and analyzed, but all were eventually discarded.

According to interviews by investigators, the tree was steadily dropping needles and the branches had started drooping. Employees of the family advised that they knew the two trees in the residence were dry, and they noted that Sandra Pyle was aware of the condition of the trees. The Pyles both wanted to keep the tree longer, because they felt like they hadn't had a chance to enjoy it, the report said.

Investigators conducted controlled burns on three trees at the ATF's Fire Research Lab in Beltsville, Maryland. Cheplak said the tests showed that a tree watered daily did not burn as quickly or as intensely as trees watered once a week.

While the study noted that the precise moisture content of the Pyles' tree was unknowable and a direct comparison wasn't appropriate, the study found it could be reasonably inferred that the amount of heat generated during the fire was "overwhelming and that it spread at an extremely rapid rate."

Donald Pyle's body was found in the collapsed debris of the great room near the spot where the 15-foot tree stood, and his wife's body was found in debris from a second-floor guest bedroom near some of their grandchildren, according to the report.

"Research has shown us in these types of situations that males tend to attempt to extinguish a fire, whereas females tend to save their family members or other victims, and that is certainly what we believe happened in this case," Cheplak said.