A high-rise blaze that killed a fire lieutenant started in a pinched electrical cord in a cluttered apartment, fire officials said Sunday, adding that the fire had been ruled accidental.

An air-conditioner cord was pinned between a bed frame and a wall in the 19th-floor Brooklyn apartment, where Lt. Gordon Ambelas became trapped while looking for possible victims, Fire Commissioner Daniel Nigro said in a statement as investigators probed the conflagration responsible for the Fire Department of New York’s first line-of-duty death in more than two years.

“Though the cause and origin of the fire has been determined, the Department’s investigation remains ongoing,” Nigro added in a statement. A pinched electrical cord can fray or otherwise become damaged enough to spark a fire if it’s near combustible items, especially if heat builds up in a tight space.

Earlier Sunday, firefighters solemnly hung flag bunting at the Brooklyn firehouse where Ambelas had worked for the last several months of his 14-year career as residents returned to the building where he had died.

The fire broke out around 9:30 p.m. Saturday in the apartment, near the top of a 21-story building owned by the New York City Housing Authority. Flames spread to the 17th and 18th floors.

The apartment was crowded with belongings, making searches difficult, the Fire Department said.

“Ambelas went into the apartment to search for life and did not come out, and by the time his brother firefighters found him, it was too late for him,” Nigro said earlier Sunday.

Fellow firefighters found Ambelas unconscious and carried him out of the building. They worked with emergency rescuers to try to revive him, but he died at a hospital, Mayor Bill de Blasio said.

“New York City and the FDNY suffered a terrible and tragic loss,” he said.

Two other firefighters and two residents were treated for minor injuries.

The Housing Authority said in a statement Sunday that it was working with firefighters on the investigation; the agency didn’t answer questions about what fire prevention devices might have been in the apartment.

A light smell of smoke hung in the air outside the building Sunday as investigators went about their work and residents came back.

Steven Jimenez, 15, had been returning from a cookout to his ninth-floor apartment when he saw flames in a 19th-floor window. As he waited outside, he watched as a bandaged Ambelas was carried out, he said.

“It looked scary … and it was scary that it happened in my neighborhood,” said Jimenez, who ultimately spent the night at a friend’s home.

Ambelas, whose fellow firefighters called him Matt, was the first New York City firefighter killed on duty since Lt. Richard A. Nappi was killed fighting a Brooklyn warehouse blaze in April 2012.

A police officer, Dennis Guerra, died this April after he and his partner were overcome by smoke and carbon monoxide while responding to a mattress fire on the 13th floor of a Coney Island public housing complex.

Ambelas, a 40-year-old married father of two daughters from Staten Island, had been promoted to lieutenant 10 months ago. He had helped the city respond to Superstorm Sandy and recover from the Sept. 11 terrorist attacks, among many other emergencies, said his fellow firefighter, friend and former roommate Eric Bischoff.

“He died a hero — that’s how he lived,” Bischoff said, calling Ambelas “truly one of the best human beings that anyone would ever want to meet.”

Ambelas was among the firefighters from Ladder 119 honored last month for helping to save a 7-year-old boy who became trapped in a roll-down gate in May. The boy was pulled 15 feet off the ground when his arm and head got stuck.

Ambelas said at the time that the incident “shows that FDNY members are always ready to help others. It was great teamwork all around.”

The boy is being raised in the neighborhood’s Satmar Hasidic Jewish community, and members of a local synagogue put up fliers Sunday mourning Ambelas’ death.

“The entire community’s heartbroken and saddened,” Rabbi Lieb Glanz said.

(AP)