Updated, 9:58 p.m. | A veteran New York City fire lieutenant died on Monday after battling a three-alarm blaze at a warehouse in Brooklyn, the authorities said.

Lt. Richard A. Nappi, 47, was commanding a hose line on the second floor of the warehouse, at 930 Flushing Avenue in Bushwick, when he began to feel ill. He emerged from the building with assistance from other firefighters and was in an ambulance driving toward Woodhull Medical Center when his “condition deteriorated,” a Fire Department spokesman, James Long, said.

The reason for his death remained unclear Monday evening, but “he definitely experienced some heat exhaustion,” Mr. Long said. In a statement, Mayor Michael R. Bloomberg said Lieutenant Nappi had had “an apparent heart attack.”

Mr. Nappi’s last medical exam with the department was in August, and he was found fit for duty, the authorities said.

“This is a very tragic day for New York City,” Mr. Bloomberg said at a news conference at the hospital. “Someone who devoted his life to keeping us safe is no longer with us.”

The fatality was the first for a New York City firefighter in the line of duty since August 2009, when a firefighter had a fatal stroke while looking for a working hydrant at a fire in Williamsburg, Brooklyn.

Eight other firefighters were injured as a result of the blaze on Monday. One firefighter was taken to Kings County Hospital Center in serious condition from smoke inhalation, while seven others sustained light injuries, the authorities said.

The fire began at about 1 p.m. and was mostly contained to the second floor of the warehouse. The building’s sprinkler system was working, and it helped suppress the blaze, Fire Commissioner Salvatore J. Cassano said.

“There are no mundane fires,” Mr. Cassano said. “As you can see from a fire that is being kept in check by a sprinkler system, it’s still dangerous.”

The warehouse is mostly occupied by American Medical Response, a private ambulance service. Other tenants include the city’s Office of Emergency Management, which stores equipment, like vehicles, at the warehouse, said a spokesman for the office, Christopher Gilbride. The building was constructed in 1987, according to city records.

The authorities said that the cause of the fire was under investigation, but that debris and cardboard boxes fueled it. Mr. Cassano said the fire had been concentrated in a part of the warehouse where cardboard boxes were stacked.

The fire grew gradually before it was brought under control at 3:52 p.m., the authorities said.

Mr. Long, the Fire Department spokesman, called it a “nasty, stubborn fire.”

On the day of his death, Lieutenant Nappi was commanding Engine 237, with four firefighters under him. He lived in Farmingville, on Long Island, where he was a volunteer firefighter. He leaves his wife, a 12-year-old daughter and an 11-year-old son, Mr. Bloomberg said.

Lieutenant Nappi joined the Fire Department in 1994, after working as a parole officer for the state and as a case worker for the Suffolk County Department of Social Services, the authorities said. Although he was not on duty on Sept. 11, 2001, he responded to ground zero, like many others, Mr. Long said. He was promoted to lieutenant four years ago.

Mr. Cassano said Lieutenant Nappi had been a top instructor at the Suffolk County Fire Academy in Yaphank, which trains volunteer firefighters, and frequently offered advice and technical guidance to younger firefighters.

“He was a leader that people would follow,” Mr. Cassano said.

Ivan Pereira and Andy Newman contributed reporting.