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

The New York City Fire Department lost 343 members at the World Trade Center. In the last 25 years, another 453 have died of 9/11-related illnesses, mainly cancer, from their service that day and during the immediate aftermath.

“It’s a heavy day for the fire department,” Commissioner Lillian Bonsignore told ABC News. “It’s not slowing down, unfortunately. We continue to live they day over and over again.”

The NYPD lost 23 members at the World Trade Center. In the last 25 years, another 469 have died of 9/11-related illnesses.

“We have to remember that the largest crime scene in the world very quickly became the largest race and recovery effort that this city certainly has ever seen,” Police Commissioner Jessica Tisch told ABC News.

She said the “vast majority” of officers working in 2001 are no longer with the department, which “makes it all the more important that we pass their legacies and stories down to the next generation.”

-ABC News' Aaron Katersky

President Donald Trump will attend the 9/11 ceremony at the Pentagon in Virginia and will not be at the New York City ceremony at ground zero, according to a White House official.

Vice President JD Vance will go to ground zero, an official said, adding that the three ceremonies at the Pentagon, ground zero and Shanksville, Pennsylvania, will all have administration officials in attendance.

Robert Schnelle remembers being the first member of the New York Police Department's K-9 Unit on the scene when the first plane struck the North Tower of the World Trade Center on Sept. 11.

Seventeen minutes later, Schnelle watched as a second plane struck the South Tower. Less than an hour later, Schnelle and his rescue dog, Atlas, hid for cover when the South Tower collapsed around them.

For the next eight months, Schnelle and Atlas remained there at what became ground zero, working 12-hour search-and-rescue shifts at the site of the deadliest terror attack in U.S. history.

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Former Sarasota, Florida, second grade student Krystal Davis remembers a wave of confusion washing over her and her classmates on the morning of Sept. 11, after a suited man whispered into the ear of the president of the United States, who was seated just in front of the students.

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Former elementary students with President George W. Bush on 9/11 reflect on the dayKrystal Davis, a student who was in the classroom with President Bush when he learned about the attacks, told ABC News, "You could see it on his face."

AP Photo/Doug Mills, File

Then-President George W. Bush was visiting Emma E. Booker Elementary School and had been listening intently to the children in Mrs. Sandra Daniels' second-grade class flexing their reading skills, when the look of admiration on his face suddenly gave way to an expression of concern.

Chief of Staff Andrew Card had just relayed to Bush that a second plane had crashed into the World Trade Center. "America is under attack," he later recalled whispering to the president.

"You could see the shift with [the president] once he got the message, and then you could see it on his face," Davis said.

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