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Frank Eltman, Associated Press
Brothers, New York Fire Department Lt. Michael Stack left, and firefighter Brian Stack, ride the back of a fire engine bearing the remains of their father, FDNY Chief Lawrence Stack, during Stack's funeral in St. James, N.Y., Friday, June 17, 2016. New York City Fire Chief Lawrence Stack died in the Sept. 11, 2001 terrorist attacks at the World Trade Center.

ST. JAMES, N.Y. — Thousands of firefighters in their dress blues stood at attention and saluted Friday as a flag-draped casket passed carrying two tiny vials of blood, the only known remains of a comrade who died in the Sept. 11 attacks.

For 15 years, the family of Battalion Chief Lawrence Stack was unable to put him to rest with a Roman Catholic funeral Mass because no trace of his body was ever found in the rubble of the World Trade Center. But the way was finally cleared when they recently discovered the blood, which the 58-year-old Stack had donated as part of a bone marrow drive for a child with cancer before the attacks.

"He gives blood for a young cancer kid — that's just Larry," said Rich Brandt, a chief with the Long Beach, California, Fire Department, who began his career as a member of the Fire Department of New York and learned under Stack when Stack was a captain in a Manhattan fire house. Brandt showed off a bracelet with Stack's name on it that he said he has worn since 2001.

"He was kind of a dad to me," Brandt said.

Stack's own two sons — both New York City firefighters — stood watch on the back of a ceremonial firetruck bearing the flag-draped casket with the vials of blood as it passed by firefighters and dignitaries standing at attention. The New York City Emerald Society Pipe Band led the procession, playing "Amazing Grace" as the casket was led into Saints Philip and James Roman Catholic Church, in St. James on eastern Long Island.

Lt. Michael Stack said during the funeral service that his father, who was a safety expert with the department, had been working in his office preparing a report on the deaths of three firefighters killed exactly 15 years ago Friday — June 17, 2001. When he learned that planes had flown into the World Trade Center, Stack raced to the scene and began helping people flee the burning towers. He was last seen assisting a man who had injured his leg when the south tower collapsed with him on the ground below.

Brandt said his friend could always be counted on to help others.

"Not only did he help people that day, but his entire career and his entire life was about helping people," Brandt said.

Theresa Stack, marking what would have been her 49th wedding anniversary Friday to the fallen firefighter, told reporters last week that she had never given up hope that his remains would be found. About a year ago, her family reached out to the New York Blood Bank after recalling both had donated blood weeks before 9/11. When vials of his blood were located, plans were made for a funeral.

Theresa Stack, who was presented with her husband's helmet as she left the church, said the family wanted to hold a public funeral "so people don't forget" 9/11.

Ray Pfeiffer, a retired firefighter with cancer, attended the funeral in a wheelchair.

"Larry was killed by a terrorist," he said. "Whether it was done 15 years ago or whether it was done yesterday, he deserves a line-of-duty funeral and he's going to get that respect. Larry was a good man."