His team from Engine 237 was in the first floor mezzanine section, and when firefighters took him out by stretcher to the street, Nappi was alert, fire officials said. But in the ambulance on the way to Woodhull Hospital, he lost consciousness and apparently had a heart attack, city officials said.
"While battling the blaze, Lieutenant Nappi overheated, suffered exhaustion and collapsed," Mayor Michael Bloomberg said at a news conference in the hospital.
"His life's work was keeping New Yorkers safe from fire. This is a very tragic day for New York. Somebody who devoted his life to New York City is no longer with us."
Bloomberg said Nappi and his wife, Mary Anne, have two children, Catherine, 12, and Nicholas, 11.
"I just spent some time with Mary Anne, who is here, to express the condolences of all the people of New York and thanked them for the ultimate sacrifice that their family made to keep New Yorkers safe," the mayor said.
The warehouse fire also resulted in another firefighter going to Kings County Hospital, where he was in stable condition, city officials said. Seven other firefighters suffered minor injuries.
Officials say 168 firefighters responded to the three-alarm fire at 930 Flushing Ave. and it was declared under control just before 4 p.m. The cause of the fire was not immediately known.
Nappi had been a Farmingville Fire Department volunteer for 20 years and was a part-time instructor at the Suffolk County Fire Academy in Yaphank, said Philip Caron, 71, of Lake Ronkonkoma, who had also been a Farmingville fire volunteer.
"I'm beside myself," Caron said. "He was a guy that would give you the shirt off his back. He was a fireman all the way. He was a volunteer in his private time and he worked for the city fire department. He ate, slept and drank fire department."
The Farmingville Fire Department hung up purple and black bunting and flew its flags at half-staff Monday night. Officials there declined to comment.
Shortly after 9 p.m. the message board outside the main firehouse lit up, saying "We are mourning the line of duty passing of F.F.D. member & FDNY Lt. Richard Nappi."
"The only good news is, he was a lieutenant who trained others," Bloomberg said, "and everything he gave to the fire department will remain."
Nappi was a graduate of Iona College, where he received a bachelor's degree in criminal justice in 1984.
The mayor said Nappi, a Bronx native, used to work as parole officer and caseworker at the Suffolk County Social Services department.
After his appointment to the FDNY on Oct. 23, 1994, Nappi worked in Engine Company 7 on Duane Street in lower Manhattan. Although he was off-duty on Sept. 11, 2001, he responded from home that morning to the World Trade Center, which was several blocks from his firehouse. In 2003, he transferred to Engine Company 302 in Queens, and after his promotion to lieutenant on Nov. 10, 2007, he was assigned to Engine 237.
Chris Pue, a retired New York City firefighter who went to Smithtown High School East with Nappi, said Nappi's response on 9/11 was documented by French filmmakers.
"They saw everything that happened," Pue said about Nappi and the other first responders. "He really didn't want to discuss it at all. Between the firemen, we had our own thing and we spoke about it, but we never shared it with the public."
In Nappi's Farmingville neighborhood, word of his loss spread quickly Monday night.