Like many of their neighbors, 5-year-old Jade Buffaloe’s family had the window air conditioner plugged into an extension cord, which was plugged into a power strip, which was plugged into the wall, authorities said.

Electricity surged through the cord Wednesday afternoon, the first-floor air-conditioning unit working feverishly to cool a Capitol Heights duplex while the girl, her father and her father’s 15-year-old brother napped upstairs, authorities said.

No one was awake when the cord overheated, authorities said. By the time Jade’s father woke to the sound of smoke alarms, flames were invading the second floor. He jumped from a window, unable to reach his sleeping daughter and brother, authorities said.

Both Jade and the brother, Rickey Cole, were pulled from the flames. But Jade was pronounced dead that night at a hospital, and Rickey died Thursday.

Jade’s father, 24, remained hospitalized Thursday in critical condition, suffering from burns and smoke inhalation, authorities said. He was not identified.

“It’s rough,” said Donte Maynard, a relative. Jade’s father “loved that girl to death.”

Cole played football at Fairmont Heights High School and was remembered as “relatively quiet and liked by many,” said Briant Coleman, a county schools spokesman. The teenager, who Coleman called a “rising star” in the school’s information technology program, was going to start 10th grade this month.

“The entire community is mourning the loss,” Coleman said.

On Thursday, the house’s brick facade was charred, and the windows were boarded up with plywood.

Officials said the fire caused about $150,000 in damage and displaced the residents of the other half of the duplex.

Firefighters were called to the duplex in the 6300 block of Carrington Court about 4:30 p.m. Wednesday. When they arrived, flames had reached the second floor, said Mark Brady, a Prince George’s County Fire and EMS Department spokesman. Jade’s father was in the front yard, severely burned, and he told rescuers that his daughter and brother were still inside.

Rescuers pulled the girl and Cole out of the burning house, but neither was conscious or had a pulse, Brady said. About 8 p.m. Wednesday, Jade was pronounced dead at a hospital. Cole was pronounced dead about 1:30 p.m. Thursday.

Brady said Jade, her father and Cole lived with the girl’s grandmother, who is the mother of Jade’s father and Cole. The grandmother, 52, was hospitalized for emotional distress, Brady said.

Neighbors said the family had lived in the duplex for many years. “This whole thing is just devastating,” neighbor Keith Siler said.

Firefighters returned to the neighborhood Thursday, going door-to-door to warn residents against plugging window air conditioners into anything other than a suitable wall outlet, Brady said. The units typically draw large amounts of electricity, Brady said, and when that electricity runs through a small cord, the cord can become a fire hazard.

“You cannot use those residential extension cords, which are pretty thin,” Brady said. “When you pull that much electricity through them, they will overheat and start a fire.”

In Jade and Cole’s house, Brady said, the window air conditioner was in the living room, drawing electricity through an extension cord. The extension cord, he said, ran to a power strip, and the power strip was connected to a wall outlet.

Derek McSwain, a cousin of Jade’s father and Cole, said the duplex’s age was somewhat to blame.

“They’re not modern,” he said. “They’re not safe.”

Eddy Palanzo and Ovetta Wiggins contributed to this report.