9/11 25th anniversary live updates: Families gather for ceremonies at World Trade Center, Pentagon, Shanksville

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."

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.

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.

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Many 9/11 first responders face lasting health effects 25 years laterABC News spoke with first responders about illnesses linked to dust, smoke and debris exposure at ground zero 25 years later— and their fight for benefits.

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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."

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ABC News has gone inside the 9/11 Memorial & Museum to witness the hallowed space that's dedicated to making sure people never forget.

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Visit 9/11 Memorial & Museum in honor of 25th anniversary of Sept. 11, 2001A quarter-century after the September 11, 2001 terror attacks, ABC News goes inside the 9/11 Memorial & Museum to witness the hallowed space that's dedicated to making sure people never forget.

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On 9/11, ABC News' Jon Karl said he raced out the door to drive to his beat, the U.S. Capitol. As he headed past the Pentagon, he said he could see smoke pouring out and police cars “going everywhere.”

When he arrived at the Capitol, it was under evacuation.

But he still “ended up spending that entire day in and around the Capitol complex,” he said, and he ended the day with a powerful interview with Hillary Clinton, then a new senator from New York.