Six Islamic extremists, including mastermind Ramzi Yousef, were convicted of carrying out the 1993 bombing.
At first, officials assumed it was an accident, a possible transformer explosion.
Kelly raced to the scene, where the bomb planted in a parked Ryder van had left a crater half the size of a football field in the trade center garage, causing more than a half-billion dollars in damage.
“I remember seeing this tremendous sea of first-responder vehicles … and smoke was coming out,” said Kelly, who was in his first stint as police commissioner.
It was only the next day that authorities concluded the blast was a bomb.
Investigators then found a vehicle identification number on a piece of the blown-up van that they traced to Mohammed Salameh, who had rented the vehicle.
On Tuesday, Knapp brought along his two daughters, ages 4 and 6, saying it was an opportunity to do more than mark a horror of the past.
“I want to show them the good that can come from it, and how people stand together and they rebuild,” he said.
Towering above the 9/11 memorial is 1 World Trade Center, the skyscraper replacing the twin towers that will be the tallest building in the Western Hemisphere when completed next year.