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Police Commissioner Ray Kelly was at 1 Police Plaza when he was first alerted to the explosion at the World Trade Center site in 1993.

A bomb ripped through an underground parking garage, killing six people. More than 1,000 others were injured.

It seems unthinkable today, but Kelly, who was the police commissioner at the time, did not immediately assume it was an act of terrorism. At first, some thought the blast might have been caused by a exploding transformer.

"Let's face it, we did not have this in our psyche -- the possibility of a terrorist attack." Kelly said.

It turned out that the bombing -- planned and executed by Islamic terrorists -- was a precursor of something much more deadly and devastating: The attacks of 9/11.

But the 1993 explosion was not the wake up call that it could have been.

"In a sense, we pay the price," Kelly said. "But governments -- federal, state and local -- were not doing enough to look over the horizon."

Some of the men involved in the plot were caught after a vehicle ID number was recovered from the wreckage.

It belonged to a rental van that had been reported stolen. When one of the men tried to get his $400 deposit back, he was arrested. His arrest led to others.

"It should have alerted us to the nature of the worldwide conspiracy. But I think the fact that it had been solved in such short order and that the people doing it were perceived to be inept," Kelly said. "It was sort of written off in many quarters and, obviously, that proved to be a mistake."

A mistake that came back to haunt the city in 2001.

The city has changed dramatically since 1993, and so have the tools the police department uses to fight terrorism.

Today, a vast network of cameras keeps a close eye on Lower Manhattan.

It is just one piece of the city's intense effort to prevent another terror attack from ever occurring again.