World Trade Center Bombing: On Feb. 26, 1993, a truck bomb placed in a parking garage of the World Trade Center exploded, killing six and injuring hundreds.

Ramzi Yousef: Identified as the mastermind behind the WTC bombing, he was captured in Pakistan in 1995 after fleeing the Philippines. In Pakistan, Yousef allegedly stayed at a guesthouse of bin Laden's. [He now is serving life in prison for the WTC bombing.]

Ahmad Ajaj: U.S. Immigration detained Ajaj, a Palestinian, when he tried to enter the U.S. in 1992. He had been on the same flight as Yousef, allegedly acted as Yousef's "mule," and was carrying false IDs.

JFK Manual: When INS agents detained Ajaj at JFK airport in 1992, he had several bombmaking books and terrorist training manuals. One reportedly bore the heading Al Qaeda, or "The Base."

Al Kifah Center: Several 1993 WTC bombing suspects were linked to New York City's Al Kifah Center -- part of the Pakistan-based Office of Services which recruited and trained volunteers to fight the jihad against the Soviets in Afghanistan. Bin Laden helped bankroll the Office of Services when it was founded.

Wadih El-Hage: A U.S. citizen, he was in a group closely related to the Al Kifah Center, and knew some WTC bombing suspects. [Several years later, el-Hage would be convicted of conspiracy to kill Americans in the 1998 bombing of two U.S. embassies.]

Phone Calls to Bin Laden: Bin Laden's name first appeared on a list of people called by WTC conspirators from a New York safe house, although bin Laden was never implicated in the 1993 WTC attack.

Bin Laden and Al Qaeda: A veteran of the Afghan mujahedeen war against the Soviets, Osama bin Laden founded Al Qaeda, or "The Base," in 1989 to extend the Afghan jihad to the rest of the world.

Wali Khan Amin Shah: Another Afghan war veteran, he was arrested in Malaysia for his role in the Bojinka plot. Bin Laden would later tell ABC News, "We were good friends." Authorities believe Shah, a Pakistani, was a bin Laden aide.

Plot to Kill Clinton: Shah reportedly told a U.S. grand jury that he plotted with Yousef to kill President Clinton during a state visit to the Philippines. The plan was abandoned when Yousef felt Clinton's security was too tight.

Abdul Hakim Murad: A Bojinka plot conspirator, Murad was captured in January 1995. A Pakistani, he told investigators that he and Yousef discussed divebombing a jet into a U.S. federal building and training Middle Eastern pilots at U.S. flight schools.

Bojinka Plot: This was Ramzi Yousef's plan to simultaneously blow up 12 jetliners over the Pacific. It was in the final stages of preparation when an explosion in a Manila apartment in January 1995 alerted authorities.

Konsojaya: Outwardly a palm oil exploration company, Konsojaya provided money to Bojinka's plotters and was a nerve center for the conspiracy. Wali Khan Amin Shah was a board member.

Mohamed Khalifa: A Saudi, and bin Laden's brother-in-law, he allegedly ran a charitable organization that laundered Al Qaeda funds. Calls were made from Konsojaya's offices to Khalifa, and he allegedly provided funds to Bojinka conspirators.

Khalid Shaikh Mohammed: Wanted for his role in Bojinka, he is a Kuwaiti and thought to be Ramzi Yousef's uncle. [By autumn 2002, investigators concluded he is Al Qaeda's chief of operations and was a central planner of the 9/11 plot.]

Jamal al-Fadl: This Al Qaeda informer filled in some gaps concerning Al Qaeda's inner workings. [He later testified in the trial of defendants convicted of attacking two U.S. embassies in East Africa in 1998.]

Attack on U.S. troops in Somalia: In October 1993, 18 U.S. soldiers in Mogadishu were killed during an ambush by Somali forces. According to Al Qaeda informer Jamal al-Fadl, Al Qaeda agents helped train the Somali attackers.

Attack on U.S. Embassies in East Africa: On Aug. 7, 1998 two truck bombs exploded almost simultaneously outside U.S. embassies in Nairobi, Kenya and Dar es Salaam, Tanzania. 224 people were killed and more than 5,000 injured.

Khalid al-Fawwaz: A Saudi national, he was indicted for his role in the embassy bombings. Investigators say al-Fawwaz was sent to London in 1994 by bin Laden to set up a communications office for Al Qaeda's various cells. [Al-Fawwaz currently is fighting extradition from Britain to the U.S.]

London Manual: A terrorist training manual found during a search of al-Fawwaz's apartment was virtually identical to one found on Ahmad Ajaj back in 1992 near the time of the first WTC bombing.

Mohamed al-'Owhali: A Saudi, convicted for his role in the East Africa embassy bombings. Al-'Owhali was supposed to die during the attack on the Nairobi embassy but fled the scene at the last minute.

Yemen Safe House: In a detailed confession, al-'Owhali led authorities to a safe house in Yemen owned by one Ahmed al-Hada. The safe house served as a kind of terrorist telephone exchange -- relaying messages to and from bin Laden in Afghanistan.

The Malaysia Meeting: In January 2000, several Al Qaeda operatives gathered for a meeting at a Kuala Lumpur condo. Malaysian agents took secret photos and passed them on to the CIA.

Nawaf Alhazmi: A Saudi, he was photographed at the Malaysia meeting. He later hijacked American Airlines Flight 77 on 9/11. [The CIA only alerted the FBI to Alhazmi's presence in the U.S. in August of 2001.]

Khalid Almihdhar: Photographed at the Malaysia meeting, he hijacked American Airlines Flight 77 on 9/11. A Yemeni national, he was the son-in-law of Ahmed al-Hada, the man who owned the Yemen Safe House.

Sufaat Condo: The Malaysia meeting was held at a condo owned by Yazid Sufaat. Sufaat has denied that he knew what was discussed at the meeting.

Attack on the USS Cole: While refueling in Aden Harbor in Yemen, the USS Cole was attacked by two men in a skiff loaded with explosives. Seventeen U.S. sailors were killed.

Fahad al-Quso: Suspected of carrying cash to fund the Cole operation and the meeting in Malaysia, he also was assigned to videotape the Cole attack. Al Quso was arrested after the Cole bombing and is in Yemeni custody.

Tawfiq bin-Atash: A Yemeni also known as "Khallad," he is suspected of playing a central role in the Cole bombing. His photograph at the January 2000 Malaysia meeting alerted the CIA to that meeting's importance.

Sept. 11 Attacks: In this totally unanticipated terrorist plot, teams of hijackers commandeered four airliners -- crashing two into the World Trade Center and one into the Pentagon. A fourth hijacked plane crashed into the Pennsylvania countryside.

Zacarias Moussaoui: Charged with being the "20th hijacker," Moussaoui had been arrested before 9/11. It was discovered that in October 2000, Moussaoui, a French citizen of Moroccan descent, stayed at the Sufaat condo (site of the Malaysia meeting) and was provided with false documents by Sufaat at the request of "Hambali." [The CIA had lost interest in monitoring the Sufaat condo by the time Moussaoui stayed there.]

Riduan Isamuddin (AKA Hambali): Investigators reportedly found out after 9/11 that Hambali, an Indonesian Muslim cleric, co-founded Konsojaya with Shah and played a key role in the Bojinka plot. He arranged for Alhazmi, Almihdhar, and Moussaoui -- all allegedly involved in the 9/ll plot -- to stay at the Sufaat condo and attended the Malaysia meeting.

Ramzi bin al-Shibh: Mohamed Atta's Hamburg roommate, he attended the Malaysia meeting, but was apparently only identified as having been there after 9/11. Bin al-Shibh allegedly wired money to a number of 9/11 hijackers, as well as Moussaoui, after he was denied a visa to the U.S. [On 9/11/02 he was captured by Pakistani police in Karachi and is currently in U.S. custody.]

Spain Meeting: After 9/11, Spanish police retraced Atta's July visit to Spain. They discovered he met with five other Arabs in the Tarragona province. One was Ramzi bin al-Shibh.

Mohamed Atta: Raised in Egypt, Atta was believed to be the ringleader of the 9/11 plot and the suicide pilot on American Airlines Flight 11. He met with bin al-Shibh in Spain two months before 9/11, and the NSA intercepted phone calls between Atta and Khalid Shaikh Mohammed before 9/11.