Winter 1990 - Spring 1991: Bin Laden and his associates relocate to Khartoum, Sudan.
Feb. 26, 1993: A car bomb is detonated under the World Trade Center in New York City. According to Federal Judge Kevin Duffy, the goal of al Qaeda mastermind Ramzi Youssef was to "engulf the victims trapped in the North Trade Tower in a cloud of cyanide gas." The explosion incinerates the gas, greatly decreasing the number of casualties. Five people die.
Late 1993 - early 1994: Al Qaeda tries to acquire uranium in Sudan to use in a nuclear device. This is the first evidence of bin Laden's plans to purchase nuclear material for an improvised nuclear device.
Evidence of this attempted transaction comes from Fadl, who defected from al Qaeda in 1996 and became a source for the FBI and CIA. He testifies in court that former Sudanese President Saleh Mobruk attempted to help al Qaeda acquire uranium of South African origin. Fadl says he heard later that the uranium, which al Qaeda acquired for $1.5 million and was tested in Cyprus, was "genuine."
1996: Zawahiri, leader of the Egyptian Islamic Jihad (which later merged into al Qaeda), is detained and released by the state security service in Russia. There is unconfirmed speculation that Zawahiri was seeking nuclear weapons or material there.
May 21, 1996: Abu Ubeida al-Banshiri, a founder of al Qaeda, dies in a ferry accident on Lake Victoria. According to testimony from senior al Qaeda officials, he was seeking nuclear material in southern Africa.
May 1996: Al Qaeda's leadership relocates to Afghanistan.
Early 1998: Zawahiri's Egyptian Islamic Jihad (EIJ) merges with al Qaeda. Zawahiri and EIJ bring technological know-how about chemical, biological, radiological, and nuclear weapons to the more ideological al Qaeda. Zawahiri takes control of nuclear and biological weapons development for the whole organization.
Before this time, high-ranking al Qaeda members had held internal discussions about the wisdom and efficacy of pursuing chemical, biological, radiological, and nuclear interests. 1998 marked the year when systematic and programmatic efforts began.
Feb. 23, 1998: Bin Laden issues a fatwa against the United States, saying, "The ruling to kill the Americans and their allies -- civilians and military -- is an individual duty for every Muslim who can do it in any country in which it is possible to do it."
Aug. 7, 1998: Al Qaeda initiates simultaneous suicide truck-bomb attacks at the U.S. embassies in Dar-es-Salaam, Tanzania, and Nairobi, Kenya. At least 230 civilians, mostly locals, die. The FBI places bin Laden on its "10 most wanted" list and starts monitoring al Qaeda closely.
Aug. 20, 1998: The United States destroys the Al-Shifa pharmaceutical factory in Khartoum, Sudan, based on suspicions that the plant might be producing the nerve agent VX for the Sudanese government and al Qaeda.
Dec. 24, 1998: Osama bin Laden states in an interview with Time's Rahimullah Yusufzai: "Acquiring [WMD] for the defense of Muslims is a religious duty."
1999-2001: Al Qaeda training camps in Afghanistan conduct basic training courses in chemical, biological, and radiological weapons for hundreds of extremists. Abu Khabab al-Masri, a chemist and top bomb-maker, and Abu Musab al-Suri (better known as Setmariam), a Spanish citizen born in Syria, conduct the training courses at the Durante and Tarnak farms.
Setmariam is captured in a raid in Pakistan on Nov. 3, 2005. The outspoken proponent of using chemical, biological, radiological, and nuclear weapons in attacks against the United States tells authorities that al Qaeda had made a mistake by not utilizing WMD on Sept. 11, 2001.
Early 1999: Zawahiri recruits a midlevel Pakistani government biologist with extremist sympathies, Rauf Ahmed, to develop a biological weapons program. He is provided with a laboratory in Kandahar, Afghanistan.
Early 1999: The head of Jemaah Islamiyah (JI), an al Qaeda-associated militant Islamist group based in southwest Asia, introduces an ex-Malaysian Army captain and California Polytechnic State University (better known as CalPoly) graduate, Yazid Sufaat, to Zawahiri.
Zawahiri starts a second, independent, parallel program to the al Qaeda Afghanistan program, with Sufaat at the helm. Neither program knows of the existence of the other; each reports to Zawahiri independently. This collaboration between al Qaeda and JI is likely the first instance of Islamist terrorist groups jointly developing WMD.
The Afghanistan program, headed by Ahmed, acquires equipment and sets up labs. Sufaat, a more trusted JI member, focuses on developing the anthrax pathogen. He has been described as the "CEO" of al Qaeda's anthrax program.
1999-2001: Al Qaeda's Abdel Aziz al-Masri conducts nuclear-related explosive experiments in the desert. He is an explosives expert and chemical engineer by training, reportedly self-taught on things nuclear.
January 2001: Pakistani nuclear scientists with extremist sympathies create the humanitarian nongovernmental organization Umma Tameer e Nau (UTN). Bashiruddin Mahmood, the former head of Pakistan's Khushab plutonium reactor, is its chair; the former head of Pakistan's Inter-services Intelligence directorate, Hamid Gul, is on its board.
Mahmood is later forced into retirement due to concerns about his extremist sympathies and reliability. He pens controversial books predicting an imminent apocalypse, offering a radical interpretation of the Quran.
June 2001: Sufaat hosts a meeting of the 9/11 attackers in Kuala Lumpur. Sufaat provides a false Malaysian address for Zacarias Moussaoui, who was arrested shortly before 9/11, to help him travel to the United States.
Before Aug. 2001: UTN's Mahmood discreetly offers to construct chemical, biological, or nuclear weapons programs for al Qaeda and the Libyan government. The United States gathers intelligence on the offers and passes it to the Libyan intelligence service office in London. The head of the London office later confirms to the United States that Libya will have no dealings with UTN.
August 2001: Zawahiri personally inspects Ahmed's completed laboratory in Kandahar. He separately meets with Sufaat for a weeklong briefing on the reportedly successful efforts to isolate and produce a lethal strain of anthrax.
Summer 2001: Mohammed Atta, an organizer and leader of the Sept. 11 attacks, allegedly meets with WMD figures, including al Qaeda's Adnan Shukrijumah. According to the FBI, Shukrijumah cases targets in New York City for possible attacks; he is later associated with multiple nuclear and "dirty bomb" plots.
A person fitting Atta's description seeks to apply for a loan to purchase a crop duster in Florida, and is refused. After 9/11, the FBI approaches every U.S. crop duster company, searching for links to terrorists.
Summer 2001: The United States detains Abderraouf Yousef Jdey, who traveled with Moussaoui from Canada into the United States. Moussaoui is detained with crop duster manuals in his possession; Jdey has biology textbooks. They might have been involved in planning a second wave of attacks for immediately after 9/11.
Sept. 11, 2001: Nineteen members of al Qaeda board two passenger planes in the United States, hijacking them and piloting them into the two towers of the World Trade Center in New York. Nearly 3,000 die.
September 2001: Al Qaeda breaks camp. Most senior operatives and their families flee Afghanistan in anticipation of an imminent U.S. invasion.
Oct. 7, 2001: The United States launches Operation Enduring Freedom, invading Afghanistan to neutralize and destroy al Qaeda and bin Laden.
Oct. 23, 2001: Pakistani intelligence services detain a long list of UTN members and associates, at the request of the U.S. government.
Sometime this month, George Tenet, the director of the CIA, meets with President Pervez Musharraf of Pakistan regarding the threat posed by UTN and the evidence that al Qaeda might be building chemical, biological, and nuclear weapons programs. Musharraf reportedly responds, "Men in caves can't do that."
Still, Musharraf agrees to work with the U.S. government to out and arrest Pakistani scientists cooperating with al Qaeda. Musharraf and Pakistan's intelligence services follow through with the promise.