ST. JAMES, N.Y. — The only known physical remains left of Battalion Chief Lawrence Stack, killed in the collapse of the World Trade Center on Sept. 11, 2001, were buried Friday in a final salute to one of New York's Bravest.

Stack perished in the 2001 terror attacks, but his body was never recovered. He was last seen tending to an injured man, a lasting testament to his character, according to his FDNY Lt. Michael Stack.

"My father stayed with the injured man, probably telling him the entire time everything was going to be alright," a tearful Stack said at the ceremony. "North tower collapsed."

Like many who died on Sept. 11, Stack's remains were lost among the ruins. His family held out hope for nearly 15 years. They almost gave up.

But recently, two vials of his blood were found. Stack had donated the blood during a bone marrow drive for a child with cancer, about 8 months before he died. They were being kept in a storage facility in Minnesota and soon sent to Stack's family.

Those vials were buried Friday with full military honors at Calverton National Cemetery following a funeral Mass at St. Philip and St. James Roman Catholic Church in St. James, Long Island.

Several thousand firefighters lined the street outside the Long Island church where his funeral Mass was held. New York City's mayor and fire commissioner were among the speakers.

Among the mourners was Al Hay, a retired FDNY Chief of Safety.

"I can only guess that (Stack's family is) getting a sense of closure and are finally getting some relief," Hay said. "I hope they're honored by this show of support."

Afterward, the casket bearing the blood vials was placed on top of a ceremonial firetruck, flanked by Stack's firefighter sons. The strains of bagpipes filled the air as the procession departed for Calverton National Cemetery.

A 6-year Navy veteran, Stack quickly rose through the ranks of the city's fire department in his 33 years on the force.

He was promoted to lieutenant in 1992, then to battalion chief in 1990, then to chief of safety command in 2000.

He was survived by his wife and two sons, both of whom are in the FDNY. One of them served during 9/11; another joined the force after that fateful day.

The Associated Press contributed to this report.