Two children playing with matches in their Seat Pleasant living room early Thanksgiving morning started the fire that killed six other children, Prince George's fire investigators announced yesterday.

The fire, which began when some school supplies were ignited and spread to a love seat cushion, quickly engulfed the ceiling of the first floor and trapped the other children upstairs, the fire officials said.

Fire Chief James M.H. Estepp said investigators had come to that conclusion only after ruling out all other theories, including an early belief that a kerosene heater in the house may have caused the fire, and after extensive interviews with members of the family living in the house.

Estepp refused to identify the two youngsters, but said they had escaped the blaze. However, the only two children to have survived the fire were earlier identified as James Q.R. Williams, 5, and his brother, Samuel A.J. Williams, 2.

Calling the announcement "a very sad finding," a somber Estepp told reporters at a hastily called news conference that "there are a thousand other reasons we would rather offer to the grieving family members who survived this tragedy, but evidence and facts which have been developed in the investigation support our findings beyond a reasonable doubt."

Ten fire investigators worked around the clock, sifting inch by inch through the charred remains of the house for three days after the fire, looking for clues as to what started one of the worst fires in the history of Prince George's County.

Three generations of the James H. Williams family lived in the two-story frame house at 203 69th St.

Seven adults were injured in the fire as they rushed from the house, most jumping to safety from first and second floor windows.

According to Estepp, the two boys began playing with matches in the living room while their grandmother, Annie Mae Williams, 47, slept on a couch a few feet away. Shortly before 8 a.m., school supplies -- a book bag and a folder -- along with a jacket caught fire, setting ablaze the foam cushion love seat.

The startled boys ran to the back of the house where their 21-year-old uncle, James Kelvin Williams, was sleeping. Fire officials said he broke out the bedroom window and helped the youngsters to safety before climbing out the window. His mother, Annie Mae Williams, escaped through the front door while her husband, James H. Williams, 57, jumped out a window in the master bedroom on the first floor.

The house, built at least 45 years ago, caught fire quickly, said investigators. Part of the reason, they said, was an extremely combustible tile ceiling, insufficient fire barriers and an open interior stairwell.

Family members were not alerted to the blaze, said fire officials, because the smoke detector did not contain a working battery.

As a wave of heat and carbon monoxide moved upstairs, family members and a neighbor sleeping on the second floor began breathing the toxic gases and soot, fire officials explained. When they awoke, most were terribly confused.

Sandra Gail Williams, 25, mother of James Q.R. and Samuel A.J. Williams, was asleep with her two other sons, 7-month-old twins Nathaniel Mark and Emmanual Mathew Williams, and her 10-year-old brother Joshua, in the bedroom directly above the living room. Investigators said she told them that when she awoke, she stood up and was hit in the forehead by a terrific blast of heat. Joshua also stood up at his bed, gasped loudly, and then fell back into his bed, she told investigators.

The twins and Joshua, who was the son of James H. and Annie Mae Williams, died.

In the rear bedroom, Sandra's sister Tracy, 24, slept with her husband Clifton Stefan Ahmad, 26, and their three children, Clifton Stefan Ahmad Jr., 5; Justin, 2, and Shenna, 1.

Tracy Williams-Ahmad went to the window and jumped, fracturing both ankles. Her husband also escaped, but their three children died of smoke and soot inhalation.

A family friend, who fire officials identified yesterday as Marvin Plater, 41, but who was earlier identified as Marvin Williams, jumped to safety from another front bedroom window.

During the painstaking search for a cause, investigators sawed out large pieces of the first floor to be tested for chemicals, analyzed the home's electrical appliances, electrical wiring, natural gas heating plant, and other potential hazards, Estepp explained.

After nearly 50 interviews with family members and neighbors, and more than 1,000 hours of work, investigators said they arrived at their conclusion on Friday afternoon.

Sgt. Timothy G. Augustine, the investigation coordinator, said that the children's playing with matches was "one of the very last things that we investigated. We were hoping that wasn't it. It's tough to think a little kid might have had something to do with something that took six lives. It's a tough thing to swallow. But it can't be disputed."

Augustine said that the theory grew stronger after talking with some family members, whom he declined to identify, and after speaking with the two boys. Investigators spent more than two hours gaining their confidence before the children unraveled the mystery.

"We just wanted to get them to tell us what happened. Then, boom, that's it. Stop there," Augustine said. "We don't want them to go through life feeling guilty."

The family was called together Friday by fire officials and was informed of the finding. The news was met by "a mixture of grief and anger," Augustine said. Some family members expected the finding, he said, while others were surprised.