9/11 25th anniversary ceremony: Families gather at WTC, Pentagon, Shanksville

The triplet sons of an FDNY firefighter, who responded to the Sept. 11 terrorist attacks, are not identical but they share a common past and will soon share a common future.

They're three of the thousands of children who lost a parent 25 years ago on Sept. 11 or in the years after due to 9/11-related illnesses.

William Gormley's sons - Raymond, Billy and Kevin - were just 3 years old when their father rushed to the twin towers on Sept. 11, 2001. While many children lost parents that day, the brothers grew up with their father, only later learning the toll his work at Ground Zero had taken on his health.

Dan Krauth spoke to the triplet brothers about carrying on their father's FDNY legacy.

More than 9,400 people have died from 9/11-related illnesses -- according to recently published data from the WTC Health Program -- which is three times higher than the 2,977 who died in the attacks in New York, at the Pentagon and in Shanksville, Pennsylvania. Data from the WTC Health Program shows about 50,000 cancer diagnoses have been certified.

Many of these people are members of the FDNY and NYPD who responded to the attacks or assisted in cleanup operations, but they also include civilians -- those who lived, worked, went to school or passed through the area.

"The truth is, 9/11 didn't end on 9/11," Michael Barasch, managing partner at Barasch & McGarry, a law firm that specializes in helping 9/11 responders and survivors get compensation and health benefits, told ABC News. "I personally lost dozens of clients, mostly firefighters and police officers, on 9/11, but I have lost thousands of clients since."

Click here to read more.

Former ABC News correspondent Ann Compton recalled being in the Florida classroom with former President George W. Bush when the Twin Towers were attacked and the frantic flight with White House staff and other reporters to safety.

"The Secret Service wanted him out of there," Compton, who was the press pool reporter that day, said of Bush. "He made a quick statement. We raced to the airport. Air Force One at that point when we reached the airport in Sarasota, the engines were roaring."

MORE | Students with President George W. Bush on 9/11 reflect on the day: 'You could see it on his face'

Air Force One was scheduled to fly back to the White House immediately but during the flight the Pentagon was attacked, she recalled.

During this time in the air, reporters had limited communications but were able to watch news feeds from the jet's TVs. It was there they saw the towers fall.

"It was horrifying. I didn't worry about Air Force One or our safety, I worried about what 20, 30,000 people in those skyscrapers, and we were watching them fall to their death," she said.

Nearly 3,000 folding chairs lined Bryant Park on Friday as part of a public memorial honoring those lost in the Sept. 11 attacks.

Each of the 2,753 chairs represented a person killed on 9/11 and was arranged facing south toward the Twin Towers that once stood in Lower Manhattan. The installation was open to the public, with visitors invited to take a flower and leave a note on a chair.

Michelle Charlesworth has the details from Bryant Park.

"It's so compelling, it's so hard, but you know they didn't win. We are still here," one visitor said while reflecting on the memorial.

Dan Biederman, founder of the Bryant Park Corporation, said members of the public were encouraged to participate in the tribute by leaving a flower or note on each of the chairs.

As visitors moved among the rows of chairs, many paused to reflect, pray and leave messages in remembrance of those lost on Sept. 11.