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

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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On the morning of Sept. 11, 2001, 34-year-old New York City firefighter Stephen Siller had just finished his overnight shift when he heard a call come in on the scanner.

"He immediately knew something was awry," so he drove to his firehouse in Brooklyn, his son, Stephen Siller Jr., told ABC News.

"When he got there, he realized his whole unit had responded [to the World Trade Center]," Siller Jr. said. "So he grabbed his gear, threw it in his truck and started to make his way to the World Trade Center. But on 9/11, they shut down every entryway into Manhattan."

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Steel from ground zero carries 9/11 sacrifice across AmericaStephen Siller Jr. honors his father’s 9/11 sacrifice by carrying World Trade Center steel across America to preserve his legacy.

ABC

He parked his car by the Brooklyn Battery Tunnel, Siller Jr. said, put about 60 pounds of gear on his back, and ran through the tunnel to the South Tower of the World Trade Center.

Stephen Siller was among 343 New York City Fire Department members killed on 9/11. He was survived by his close-knit siblings, his wife Sally, and his five children, including Stephen Jr., who was 9 months old.

Siller Jr., now 25, doesn't have his own memories of his father, but he's poured his energy into honoring those who died on 9/11.

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When American Airlines Flight 77 slammed into the Pentagon, FBI Special Agent Jeffrey Miller rushed to the scene, spending weeks on site recovering evidence from the burned wreckage.

Now, two and a half decades later, he is one of the many FBI agents that are suffering from the long-lasting effects of that day.

So far more than 30 FBI personnel have died from cancer or other illnesses the FBI says are related to exposure to toxic material at the 9/11 sites. Miller says he personally knows at least a dozen of them.

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