Loaded with around 1,300 pounds of urea nitrate (an explosive material made from fertilizer) and hydrogen gas cylinders, along with cyanide, the bomb exploded inside a yellow Ryder van parked on the B-2 level of the parking garage under the North Tower. The massive blast killed six people near the bomb site and injured more than 1,000, with most of those suffering from smoke inhalation during evacuation from the towers.

Within minutes of the explosion, members of the New York Joint Terrorism Task Force (JTTF) headed to the World Trade Center, where they would coordinate an investigation including the Federal Bureau of Investigation (FBI) and New York Police Department (NYPD) among other federal, state and municipal government agencies. The FBI and JTTF had been tracking Islamic fundamentalists in the city for months before the attack and immediately suspected this was an act of terrorism. The day after the attack, agents searching the wreckage found several parts of a vehicle that apparently exploded from the inside out. Two of the pieces showed a vehicle identification number (VIN), which investigators traced to a van that had been reported stolen on the day before the attack at a rental agency in Jersey City, New Jersey.

When Mohammed Salameh, the man who had rented the van, returned to the rental agency on March 4 to try to get his $400 deposit back, an FBI team arrested him. Things unfolded quickly from there: FBI investigators found bomb-making chemicals that matched evidence found at the World Trade Center in a self-storage unit in Jersey City, while The New York Times received a letter claiming responsibility for the attack from a group called the Liberation Army, Fifth Battalion. A search of Salameh’s apartment led to the arrests of three more suspects, including Nidal Ayyad (whose DNA matched saliva on the letter’s envelope), Mahmud Abouhalima and Ahmed Ajaj. Investigators questioned another suspect, Abdul Yasim, but released him due to a lack of evidence; he subsequently fled the country and has never been captured.

While Salameh, Ayyad, Abouhalima and Ajaj were tried, convicted and sentenced to life in March 1994, the man identified as the plot’s mastermind had escaped to Pakistan immediately after the bombing. Over the next two years, Ramzi Yousef took part in various other terrorist actions, including planting a bomb on a commercial airplane in the Philippines in order to test a larger plot involving explosions on as many as a dozen U.S. planes. Captured in February 1995, Yousef was extradited to New York City, tried and found guilty of both the bombing and the Manila plane plot, code-named “Bojinka.” He was sentenced in January 1998 to life in prison plus 240 years, with the judge factoring in combined life expectancies of the six people killed in the 1993 bombing. Yousef was unapologetic, claiming he wanted to punish the United States for its role in providing aid to Israel. “Yes, I am a terrorist and proud of it,” he told the court.

The arrests of Salameh, Abouhalima and Ayyad led FBI investigators to a Brooklyn mosque all three had attended, and to Sheikh Omar Abdel Rahman, a radical Sunni Muslim cleric who had emigrated to the United States in the early 1990s. Abdel Rahman, who lost his eyesight at a young age, espoused a fundamentalist brand of Islam that condemned secular Muslims, Western materialism and U.S. support for Israel and Egypt. In the early 1980s, he had been tried and acquitted twice for instigating the assassination of that country’s president Anwar Sadat.

In June 1993, as part of an ongoing surveillance operation, an FBI camera caught a group of men constructing a bomb in a garage in Queens, New York. Authorities subsequently arrested Abdel Rahman and nine of his followers and charged them with planning to simultaneously bomb the United Nations headquarters, the George Washington Bridge, and the Holland and Lincoln Tunnels, among other targets. They were found guilty and sentenced to life in prison in late 1995. Even behind bars, Abdel Rahman continued to exert a powerful influence on radical Muslims. Peter Bergen, a journalist and biographer of Osama bin Laden, described him as the “spiritual guide of 9/11.”

By early 1996, U.S. authorities had determined that Ramzi Yousef’s uncle, Khalid Sheikh Mohammed, had plotted with his nephew to bring down the planes in the “Bojinka” plot and wired him money before the first World Trade Center bombing. But KSM (as he was known) evaded capture and rose in the ranks of the militant Islamic group al Qaeda to become one of bin Laden’s top lieutenants. As the “principal architect” of the terrorist attacks of 9/11, KSM succeeded where his nephew had failed, orchestrating the destruction of the twin towers and the mass murder of thousands of civilians on U.S. soil.