9/11 was such an overwhleming and horrific event that it has overshadowed the first such attack on the WTC. But in many ways that first attack was almost like a dry run for the later and more deadly one.
It was carried out by people with links to Al-Qaeda, the main instigator regarded it as a justified act of terrorism and in the aftermath of the attack the lack of fully effective co-operation between various Government agencies was on full display.
The attack happened on February 16th 1993. Two of the conspirators drove a truck filled with 590K of explosives into a public parking garage beneath the WTC, set a 20 foot fuse and walked away.
The resulting explosion cut the electricity to the WTC and forced smoke from the explosion into the stairwells up to the 93rd floor of both buildings. In addition to people injured or killed because they were close to the explosion, people were trapped in elevators, and many others in the twin towers had injuries from smoke inhalation. In all 6 people were killed and 1,042 were injured. In addition a number of first responders were also injured.
While the affects of the bombing were bad they were not nearly as catastrophic as had been planned. The intention was to park the truck with the explosives close to one of the main supports of the one of the WTC buildings and bring it down. When that happened it was hoped it would fall onto the second tower and bring it down also. Fortunately the placement of the truck was off and the none of the WTC’s support columns were damaged sufficiently to endanger the stability of the buildings.
In all 6 people planned and carried out the attack and ultimately all of them were caught and convicted. Prior to the attack, the main conspirator Ramzi Yousef, had mailed threatening letters to NYC newspapers saying an attack would take place unless certain demands were met. It is unclear if any action was undertaken as a result of those letters, prior to the attack.
Initially bomb experts from the ATF (Bureau of Alcohol, Tobacco and Firearms) the FBI and the NYPD were invovled in the investigation. Later the CIA and the DSS (U.S. Diplomatic Security Services) also became involved and DSS officers were the ones who nabbed the two main conspirators, though the FBI got the credit at the time.
Though all these agencies played an important role, there was considerable murkiness around the nature of the role the FBI played prior to the attack (see the 2nd video) as they had double agents imbedded in the circles in which the conspirators moved. As a result there have been endless conspiracty theories put forth about exactly why things happened the way they did.
Security was certainly beefed up following the attack especially in the underground garages and also by the time 9/11 took place much better evacuation procedures were in place. Whether the agencies did a collective debrief and changed the protocols for interagency co-operation is unknown.
Then as now the New York Port Authority has ultimate responsibility for the security of the WTC.