Bureau of Alcohol, Tobacco, Firearms and Explosives agents also concluded that the fire, which started in the home's great room, was an accident, caused by a faulty power outlet set into the floor near the tree.
The rapid combustion of the tree - combined with wide-open chambers of the mansion - supplied ample oxygen to fuel the blaze, according to ATF investigators.
"While the explanation that has been shared with us today does not bring solace, it does start us down the long road to acceptance," read a statement issued today from the Boone and Pyle families.
It took almost a week for six bodies to be found in the burned-down mansion. All those who were thought to be inside are believed to be accounted for, according to fire department officials.
The 16,000-square-foot mansion was owned by tech executive Don Pyle and his wife Sandra.
According to relatives, the Pyles had four of their grandchildren -- Alexis (Lexi) Boone, 8; Kaitlyn (Katie) Boone, 7; Charlotte Boone, 8; and, Wesley (Wes) Boone, 6 -– over at the time of the four-alarm fire.
Crews started going through the wreckage last Wednesday, according to ABC News affiliate WJLA, a process that ATF investigators said could take weeks.
The house was "built more like a commercial structure," said Anne Arundel County Fire Capt. Russ Davies, adding that searching is a "time-consuming process."
Initially, the fire had been handled as a criminal investigation.
Relatives described the Pyles as loving grandparents nicknamed “Pop-Pop” and “Dee-Dee.” The night before the blaze, Don and Sandra treated their four grandchildren to a special outing to Medieval Times, according to a family spokeswoman -– even taking them to Target beforehand to pick up costumes for the occasion.
"It is our hope that each of you are strengthened in your resolve to cherish your family, friends, and good times. With life so fleeting, make every day and every moment a special time with those you love," the statement from the victims families continued. "We believe that life is about making memories. As we work through our pain and loss, the memories we made with our family will sustain us."
Authorities said at today's news conference that they would conduct additional tests and analysis.