For the NYPD cops who responded to ground zero, sifted through the Fresh Kills landfill in Staten Island or escorted fallen first responders to the morgue, the memories of September 11, 2001, are as vivid as ever, each officer grappling with their emotional scars in their own way.

"If I talk in the third person, like I'm not there, then I can convey the story," says now-retired Inspector Timothy Pearson.

"But to this day I start getting choked up just thinking about it. Because when I do, I see the fear and I feel the fear."

Pearson, then a captain running a housing command based in East New York, Brooklyn, drove in that morning with about a dozen other cops. They were able to escort hundreds of people to safety before the South Tower pancaked and collapsed.

But he laments the loss of a friend, Officer John Perry and is still haunted by the sight of those who jumped from the burning buildings out of desperation.

"As I'm looking up I see what I thought was one of the strangest things," Pearson recalled. "I'm wondering why there were birds flying down through the smoke."

"Then I realised they were people coming down - people."

Before thousands of cops arrived at ground zero, Chief of Department Joseph Esposito was already there, racing over from One Police Plaza.

Over the radio he ordered the White House be notified, reversed an order by another commander who wanted a police helicopter to land on one of the towers, and told supervisors in the outer boroughs to mobilise their officers to safeguard key locations because it wasn't yet clear if the terrorists would strike outside Manhattan.

Esposito, the department's highest-ranking uniform officer, then spent that day helping get to safety as many people as possible.

Physically and mentally exhausted, he went home that night long enough to kiss his wife and change his clothes. For weeks after, he returned every day to ground zero to take a spot on the bucket brigade, searching for remains.

"After a while, I'm saying to myself, 'Why are we doing this?' There's nobody alive,'" Esposito recalled. "But I realised it was therapy for the people who were there."

And for Esposito, who fought back tears as he spoke.

He had become friendly with John O'Neill, the former FBI terrorism expert who just two weeks earlier had started his new job as head of security at the World Trade Centre.

Esposito ran into O'Neill that morning.

The next time he saw him was on September 21, when he helped carry O'Neill's body from the wreckage.

Detective John Maier was among a number of detectives who watched the towers collapse from the roof of Russo's On the Bay, the Howard Beach, Queens, restaurant where they had gathered to campaign for candidates whose primaries were postponed that day.

Maier was later assigned to the Staten Island landfill where the debris from ground zero had been transported, and where most days were spent in virtual silence as cops searched for body parts and mementos - a picture, an ID, a desk souvenir - that might bring closure to a loved one.

Another detective, Paul DiGiacomo, made his way to ground zero, where he was struck by how fake the carnage looked.

"The body parts that you did see - they didn't even look real because they were covered with dust," DiGiacomo said. "Usually, at a scene, there's blood. But there was no blood.

Today, DiGiacomo is president of the union, which means learning every day about another detective sick or dying of cancer contracted during countless hours breathing in toxic fumes at the World Trade Centre site.

DiGiacomo himself suffers from acid reflux and sinusitis but he's already experienced a loss beyond compare - his sister Diane DiGiacomo, an ASPCA investigator who helped rescue pets left behind in apartments near the towers, died of cancer in 2015.

But some stories have ended better.

Detective Patricia O'Connor, who helped lead survivors out of the Brooklyn Battery Tunnel that day and later spent time at ground zero, beat back skin cancer.

"Insurmountable" is how she described the task at the pile, but when she was later assigned to the morgue she realised all was not hopeless.

"My motivating factor was I wanted to match someone's DNA to this piece of flesh, to be blunt, so that someone can put their loved one to rest," she said. "When the world is falling apart ... you had to stay focused on something."

"And for me staying focused on bringing closure to a family was what kept my mind sane."

Australian Associated Press