Twenty years ago today, a bomb exploded in the parking garage of the World Trade Center just after noon. Six people were killed and over 1,000 were injured. Yesterday Police Commissioner Ray Kelly, who rushed to the scene on that day, told NY1 this first bombing should have been a wake-up call, "Let's face it, we did not have this in our psyche—the possibility of a terrorist attack."

As History.com explains about the attack: "A crater 60 feet wide [was left], causing the collapse of several steel-reinforced concrete floors in the vicinity of the blast. Although the terrorist bomb failed to critically damage the main structure of the skyscrapers, six people were killed and more than 1,000 were injured. The World Trade Center itself suffered more than $500 million in damage. After the attack, authorities evacuated 50,000 people from the buildings, hundreds of whom were suffering from smoke inhalation. The evacuation lasted the whole afternoon." Here's some news footage from the day:

Amid the rubble was a vehicle identification number for a Ryder truck. On March 4, 1993, when Mohammad Salameh tried to get the $400 truck deposit back after claiming it had been stolen, the FBI arrested him. More arrests followed, but the mastermind, Ramzi Yousef, wasn't arrested until 1995. According to the FBI, "We later learned from Yousef that his Trade Center plot was far more sinister. He wanted the bomb to topple one tower, with the collapsing debris knocking down the second. The attack turned out to be something of a deadly dress rehearsal for 9/11; with the help of Yousef’s uncle Khalid Sheikh Mohammed, al Qaeda would later return to realize Yousef’s nightmarish vision."

It's also believed that sheik Omar Abdel Rahman, the blind cleric, encouraged his militant Muslim followers to bomb the World Trade Center. An FBI informant, Emad Salem, who aided with the capture of terrorists, told NBC New York that the feds must not release Rahman, "He will kill Americans. He will kill anyone who disputes what he says with a fatwa."