Mourners packed a Long Island church Friday and spoke of bravery, and forgiveness, at the funeral of an ATF agent killed by friendly fire while trying to stop a robbery.

A silver hearse carried the body of John Capano to a church in Seaford, the quiet, middle-class Long Island town where he grew up and where he died.

Thousands of law enforcement officers stood at attention as it passed. Pipers played "Amazing Grace" as the casket was carried into the chapel.

During the service, U.S. Attorney General Eric Holder called Capano "a true patriot" who possessed "extraordinary courage."

As a senior special agent for the Bureau of Alcohol, Tobacco, Firearms and Explosives, Capano had traveled to global hot spots like Afghanistan and Iraq, sharing his expertise investigating bombings, but he died close to home because of a mistake.

The 51-year-old father of two was filling a prescription for his ailing father at a neighborhood pharmacy when a man announced a robbery. Capano followed the man out the door, where they struggled on the sidewalk.

In the chaos, a retired Nassau County police lieutenant, who had also rushed to the scene alongside an off-duty police officer, mistook the agent for the robber and fired a shot that killed him.

Capano's older brother, James, asked mourners during the service to pray for those lawmen, too, saying they had also been acting on heroic instincts.

"We all know I could stand up here and talk for hours about John," he said. "Let us not forget others responded that day to stop a crime. Please remember, they have family, and acted as John did. However this turns out, I ask that we pray for them, too."

Capano, an agent for 23 years, was the first ATF agent to be killed by gunfire in the line of duty since the raid on a religious cult compound in Waco, Texas, in 1993.

The funeral was held at the St. William the Abbott Church, where the Capano family had gathered just a few weeks ago to say goodbye to the agent's late mother, who had died of cancer.

As mourners exited the church Friday, eight police helicopters honored the agent with a flyover.

The acting director of the ATF, Todd Jones, also spoke at the funeral.

As the crowd of mourners waited for the service to begin, friends of the slain agent said they still couldn't believe what happened.

"I'm still in shock. I'm still trying to figure this out," said ATF agent Gerry O'Sullivan, who knew Capano for 15 years. "John did what most trained law enforcement officers would do. And he did what he thought was best in protecting the public from violent criminals."

Investigators are still reconstructing what happened in the moments after the robbery. The robber, James McGoey, turned out to be carrying a pellet gun. He was shot and killed in the fracas by the off-duty New York City police officer who had responded alongside the retired lieutenant, Christopher Geraghty.

Geraghty's attorney said in an interview Thursday that the 54-year-old opened fire after a shot whizzed past him, mistakenly thinking that Capano was the robber.

"He's devastated," attorney Brian Davis said of Geraghty. He said he is aware the Capano family does not blame him for the tragic outcome and has sent a note of condolence, but said the shooting "will be with him for the rest of his life."

Capano, who grew up in Seaford and lived in nearby Massapequa, is survived by his wife and a son and daughter.

A spokesman for the ATF said Thursday that although Capano was not scheduled to be working that day _ he was on bereavement leave because his mother had died in mid-December _ his death is still considered to have happened in the line of duty. Spokesman Steven Bartholomew said that when Capano took action to stop the robbery in progress, he was acting as an ATF agent.

The shooting was the second friendly fire confrontation in Nassau County in the last year. A Nassau police officer in plainclothes was shot to death in March by a transit authority officer in Massapequa Park.

It was the second deadly holdup in a pharmacy on Long Island in 2011. In June, a gunman killed two store employees and two customers before fleeing with a backpack filled with painkillers.