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."

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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.

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.

ABC News Chief Global Affairs Correspondent Martha Raddatz was reporting from the State Department and her husband, also a journalist, was at the Pentagon on 9/11.

When the Pentagon was attacked, Raddatz said she and her cameraman walked to Memorial Bridge where they saw the flames shooting out of the building.

“By then I knew which side of the Pentagon had been hit, I knew my husband was on the other side. I just felt like I knew he was OK,” she said. “But to watch from Memorial Bridge, where we were all day, and imagine that that fourth plane was going to hit the Capitol, the White House. I could hear the panic in our people from the White House. ... Fighter jets going down the Potomac River and just streams of men and women in uniform crossing into Washington, D.C... to get ready for whatever they needed to get ready for.”