On Tuesday, Americans paused to remember the horrors and heroism seen on September 11, 2001, and now the man who led the NYPD through that harrowing day and the days that followed is sharing his vivid memories of that day.

Bernard Kerik served as NYPD commissioner from August 2000 until the end of 2001.  He is now the author of the fictional thrilled, "The Grave Above the Grave," which is about a New York City Police Commissioner dealing with a future terrorist attack.

Seventeen years ago on 9/11, Kerik has just completed a workout when his chief of staff told him a plane had hit one of the World Trade center towers.  Early speculation was that a small private plane had gone off course, but Kerik knew that wasn't right as he looked out the window at the burning skyscraper just a quarter mile away.

Kerik immediate called Mayor Rudy Giuliani and they planned to meet at 7 World Trade Center.  Kerik arrived first and personally witness the desperation of the people trapped in the tower.  When his driver attempted to turn onto Vesey Street, an officer stopped them to explain why they couldn't continue.

"A police sergeant came up to my vehicle and said, 'Look, you can't turn into the block.  They're jumping.'  Immediately, I didn't grasp what he was talking about.  I stepped out of the vehicle and for the next several minutes I watched people jump from the top of Tower 1.  They were landing on Vesey and they were landing in the courtyard between the two buildings," said Kerik.

Giuliani arrived about five minutes later, but just before he arrived, the second plane hit the south tower.  Kerik immediately knew that meant the city was under attack.  Not knowing how many more planes may be coming, he ordered officers to evacuate high profile targets like city hall, the Empire State Building and the United Nations.  

"We basically shut down the city and I think that was the first time in the city's history that it was actually closed," said Kerik.

The towers were attacked just 17 minutes apart.  For the next hour, first responders raced through the building to get people out.  He considers that the greates rescue operation in American history.

"They took 20,000-25,000 people out of those buildings and the immediate surrounding area.  But they also evacuated more than a million people out of Manhattan and into the four other boroughs and into New Jersey," said Kerik.

Kerik and Giuliani were in 7 World Trade Center when Giuliani learned from the White House that a plane had struck the Pentagon.  At that same moment, the south tower imploded.

"It felt like a freight train was coming through the side of it.  The door slammed open and my chief of department, Joe Esposito, came in and yelled, 'Everybody get down.  The building's coming down'  That was Tower 2.  It imploded just about on top of us," said Kerik.

What Kerik saw police and firefighters do next exceeds the definition of bravery.  He says every officer who didn't die in the Tower 2 collapse rushed into Tower 1 to save as many lives as possible.  Tower 1 imploded less than 30 minutes later, taking more lives of New York City's finest.

Kerik is now active in two ways in the wake of 9/11.  First, he is pleading with Congress to fund efforts to support more than 10,000 Ground Zero personnel who contracted life-threatening illnesses from the toxic air on site.  

He is also urging political leaders and the public to stay vigilant.  He says terrorists flying planes into buildings is unlikely to happen again, but the threat of five or six groups of terrorists targeting an elementary school or some other soft target would "put this country into a manic depression."

"The threat is still here," said Kerik.  "The threat is something we have to deal with.

Listen to the full interview with Bernard Kerik to hear more of his first-person account of 9/11 and the fight that still continues.