Ali Mohamed, a U.S. citizen and Al Qaeda member, is arrested in the U.S. Soon after, he begins cooperating and admits he took pictures of the Nairobi embassy and showed them to bin Laden. On Oct. 20, 2000, Mohamed tells a judge, “Bin Laden looked at the picture of the American embassy and pointed to where a truck could go as a suicide bomber.”

At a bail hearing for Wadih el-Hage, the U.S. Attorney claims el-Hage had links to El Sayyid Nosair and Mahmud Abouhalima, both convicted in the 1993 World Trade Center bombing.

The U.S. government releases its indictment against bin Laden, Muhammad Atef and other members of Al Qaeda.

Khalid Shaikh Mohammed allegedly visits Hamburg, Germany. After Sept. 11, 2001, U.S. officials suspect he might have met with an Al Qaeda cell in Hamburg.

Bin Laden is wanted for murder of U.S. nationals outisde the U.S.; conspiracy to murder U.S. nationals outside the U.S.; and attack on a federal facility resulting in death.

View the FBI's Most Wanted poster.

Jordanian officials intercept a phone conversation between Abu Zubaydah, a senior Al Qaeda lieutenant, and members of a Jordanian cell planning a plot referred to as “the day of the millennium.” The Jordanians conduct raids and discover explosives and a plan to blow up the Radisson Hotel in Amman and other sites.

The CIA intercepts a phone conversation at a Yemeni house that is an Al Qaeda logistics center, which they had learned about from Mohamed al-'Owhali, who was convicted in the embassy bombing case. The house is owned by Ahmed Al-Hada, a Yemeni citizen.

The callers discuss an upcoming January 2000 meeting in Kuala Lumpur. Officials learn that Khalid Almidhar, a Yemeni citizen believed to be the son-in-law of Al-Hada, and Nawaf Alhazmi, thought to be a Saudi national, will be attending the meeting. Both Almidhar and Alhazmi will later be hijackers of American Airlines Flight 77 on Sept. 11.

Algerian native Ahmed Ressam is caught entering the U.S. with 130 pounds of explosives at the Canadian border at Port Angeles, Washington. Ressam had links to other Al Qaeda militants and trained in an Afghanistan Al Qaeda camp. Authorities eventually learn his intended target was Los Angeles International Airport.

Read more about Ahmed Ressam on FRONTLINE's "Trail of a Terrorist" Web site.

Several individuals linked to Al Qaeda meet in Kuala Lumpur, Malaysia. At the CIA's request, Malaysian agents photograph the meeting. Nawaf Alhazmi and Khalid Almidhar (both later hijackers on Sept. 11) attend the meeting at the condo of Yazid Sufaat. Tawfiq bin-Atash (AKA "Khallad") -- who once headed bin Laden's bodyguards and would later become a suspect in the attack on the USS Cole -- also attends the meeting.

It is later reported that Riduan Isamuddin (AKA "Hambali"), a militant Islamic preacher -- who would eventually be suspected of having had a role in the “Bojinka” plot -- also attends the meeting. Officials later also claim Ramzi bin al-Shibh -- a former roommate of Sept. 11 hijacker Mohamed Atta -- was also photographed at the meeting.

Fahad al-Quso, who is later arrested for his role in the 2000 USS Cole attack, is a suspected “bag man” who carried money to finance the meeting. After Sept. 11, al-Quso denies having attended the Malaysia meeting. However, he would admit he met at other times with bin Atash, Alhazmi and others who were at the Malaysia meeting.

[Note: All the individuals in the photographs are not be identified by intelligence officials until later, and the importance of this Malaysia meeting would not be known until after the Cole investigation began to focus on bin-Atash. It's unclear at what point the CIA began to recognize the meeting's importance. Sources would later tell FRONTLINE that FBI agents looking into the Cole attack were not fully told about the meeting or shown pictures of those photographed until the summer of 2001.

Read more about the CIA and FBI communication failure.]

2004 Update: "Hambali" and Tawfiq bin-Atash now in U.S. custody; Fahad Al-Quso now in Yemeni custody - see details.

A cell of Yemeni terrorists try bombing the USS The Sullivans in Yemen's Aden Harbor, but fail when their overloaded skiff sinks. Investigators do not discover the attempt on the USS The Sullivans until after the USS Cole had been successfully attacked by the same cell in October of 2000.

Nawaf Alhazmi and Khalid Almidhar fly into Los Angeles from Bangkok. A later article in Newsweek reports that the CIA knew that Alhazmi was on the plane, but did not know about Almidhar. Neither individual is tracked once they entered the country. Both become suicide hijackers on Sept. 11.

The Phoenix office of the FBI begins to investigate Zakaria Mustapha Soubra, a Phoenix flight school student suspected of having ties to Al Qaeda.

Between August 2000 and early 2001, Italian investigators begin to record the conversations of Abdulsalam Ali Ali Abdulrahman. According to a later report in the Los Angeles Times, in one of the conversations Abdulrahman tells Abdelkader Moahmoud Es Sayad, an Egyptian suspected terrorist, that planes could be used as weapons against the U.S. According to the article, the FBI was aware of the conversations, but did not receive any reference to planes being used as weapons.

Ramzi bin al-Shibh, a Yemeni who was Mohamed Atta's former roommate, applies four times for a visa to enter the U.S., but is denied each time. U.S. officials later allege that he was supposed to take part in the Sept. 11 plot, but at the last minute was slotted to be replaced by Zacarias Moussaoui.

Zacarias Moussaoui, a French national, visits Malaysia and stays at Yazid Sufaat's condo after Riduan Isamuddin, (AKA "Hambali") asks Sufaat to put up Moussaoui. Sufaat, a Malaysian, also provides Moussaoui with fake identification papers. Also at Hambali's request, a company owned by Sufaat purchases explosives for an attack on foreign embassies and other targets in Southeast Asia. The plot is foiled after Sept. 11, when a videotape of potential targets is found in an Afghanistan safe house.

Two men in a skiff pull alongside the American destroyer and detonate an explosive that rips through the hull and kills 17 U.S. sailors. Yemeni authorities quickly capture and start identifying suspects. Among them is Tawfiq bin-Atash, former head of bin Laden's bodyguards. The CIA eventually comes to realize that bin-Atash had been photographed at the earlier Malaysia meeting and begins to reexamine those photographs. A later document released by the British government would claim that Mohamed al-'Owhali, convicted for his role in the 1998 embassy bombings, links two of the Cole suspects to the embassy bombings.

2004 Update: Tawfiq bin-Atash now in U.S. custody - see details.

ABC News' John Miller reports authorities have found a number of connections between the Cole attack and Osama bin Laden, including telephone records of calls between the bombers of the Cole and an Al Qaeda cell in East Africa. Yemeni officials arrest Gamal Al Badawi, a suspect who admits he fought with Al Qaeda forces in Bosnia. Fahad al-Quso, in custody, apparently carried $5,000 from an associate of bin Laden to Cole conspirators. Lastly, Miller reports Yemeni authorities suspect Abdul Al-Nassir both organized the Cole attack and also recruited bombers for the attack on the embassies in East Africa in 1998.

2004 Update: The Fahad Al-Quso case - see details.

Between January and August of 2001, the Federal Aviation Administration issues 15 advisories to airlines and airports warning that terrorists could try to hijack or destroy American aircraft.

Italian authorities record Abdelkader Mahmoud Es Sayed, an imam in Italy, talking about fraudulent documents for “the brothers going to America.”

Richard Clarke, the National Security Council counterterrorism chief, sends a memo to Deputy National Security Advisor Stephen Hadley warning that Al Qaeda sleeper cells within the U.S. are “a major threat.” Clarke also advocates targeting Al Qaeda training camps in response to the Cole bombing.

The Washington Post reports on this date that investigators in Yemen believe that people in custody are tied closely to Al Qaeda. An anonymous Bush administration official tells The Post, “There is no question that Al Qaeda was involved in this attack.”

Instructors at an Arizona flight school become suspicious about a students who speaks English poorly and has limited flying skills. They report him to the FAA. The student, Hani Hanjour, a Saudi, later pilots a plane into the Pentagon on Sept. 11.

Washington reportedly receives a “specific threat” that Al Qaeda may attempt to attack American targets in the Middle East or Europe.

This warning to airlines states: “The FAA does not have any credible information regarding specific plans by terrorist groups to attack U.S. civil aviation interests... Nevertheless some of the current active groups are known to plan and train for hijackings... The FAA encourages U.S. carriers to demonstrate a high degree of alertness.”

The State Department warns that American citizens overseas may be targeted by Al Qaeda.

Mohamed al-'Owhali, Khalfan Khamis Mohamed, Mohammed Saddiq Odeh, and Wadih el-Hage are convicted on charges including conspiracy to kill Americans, which stem from the 1998 embassy bombings.

The State Department re-issues a visa to Khalid Almidhar, a Sept. 11 hijacker who was present at the January 2000 Malaysia meeting.

Citing a security threat, the FBI pulls investigators out of Yemen. This same month, the FAA issues more warnings to airlines. The NSC's Richard Clarke later tells FRONTLINE that those warnings were “absolutely” related to Al Qaeda operatives discussing what would eventually become the Sept. 11 attacks.

By late June, intelligence experts are extremely concerned about the possibility of an imminent attack.

The U.S. embassies in Senegal and Bahrain are shut down and the State Department issues a new worldwide caution.

Yemeni authorities thwart an attack on the U.S. embassy in Sana, Yemen.

U.S. Central and European Command impose “Force Protection Condition Delta” because of concerns about a terrorist attack.

National Security Adviser Condoleezza Rice is warned during an intelligence briefing that an Al Qaeda attack is “highly likely.”

Sources close to the Cole investigation tell FRONTLINE that during the summer of 2001 the CIA informs the FBI about the Malaysia meeting and shows them a picture of one or two of the Cole suspects. The FBI reportedly identifies Cole suspects Tawfiq bin-Atash and Fahad al-Quso. Al-Quso would later deny that he made it to the meeting and claims that the individual in the picture just looked like him.

Khalid Almidhar re-enters the U.S. through JFK airport. He later meets with Mohamed Atta.

Mohamed Atta meets with his former roommate Ramzi bin al-Shibh in Spain along with a number of other Arab men. Only after Sept. 11 do officials discover the meeting and conclude that bin al-Shibh also may have been at the January 2000 Malaysia meeting.

The FBI's Phoenix office sends a memo to FBI headquarters requesting officials initiate a nationwide investigation of flight schools. The memo warns that bin Laden supporters may be attending flight schools in the United States.

The FBI warns that the conviction of Ahmed Ressam for the millennium plot to detonate a bomb at Los Angeles airport could lead to retaliatory terrorist attacks.

The FAA issues yet another warning to airlines that terrorists could be planning to hijack American airlines.

According to a later report by the British government, “In August and early September [2001] close associates of bin Laden were warned to return to Afghanistan from other parts of the world by Sept. 10.”

The FAA warns airlines that terrorists may use weapons modified from everyday objects.

Minneapolis FBI agents pick up and arrest Zacarias Moussaoui on immigration charges for overstaying his visa, but agents are concerned he could be a terrorist. The Minneapolis office tries unsuccessfully to secure either a criminal search warrant or an intelligence warrant to search Moussaoui's belongings. After Sept. 11, a federal indictment would claim that Moussaoui was in possession of two knives, a flight manual for a 747-400, fighting gloves and shin guards, and an aviation radio.

The CIA cables the FBI, warning that Khalid Almidhar and Nawaf Alhazmi are inside the U.S. and are suspected terrorists because of their presence at the January 2000 Malaysia meeting.

They say Moussaoui has been linked to Al Qaeda.

Ramzi bin al-Shibh, the former roommate of Mohamed Atta, who had attended both the July meeting in Spain and the January 2000 Malaysia meeting, leaves Germany for Afghanistan.

Hijackers alleged to be members of Al Qaeda take control of four airliners and crash two into the World Trade Center, and one into the Pentagon. A fourth hijacked plane crashes into the Pennsylvania countryside. The attacks kill more than 3,000 people.

U.S. investigators discover evidence they believe links Khalid Shaikh Mohammed -- wanted and still at large for his role in the 1995 Bojinka plot -- to the Sept. 11 hijackings. A few weeks later, U.S. officials say they believe that Khalid Shaikh Mohammed is the operations chief of Al Qaeda. It is also later reported that the National Security Agency had intercepted telephone conversations between Mohamed Atta and Khalid Shaikh Mohammed before Sept. 11, but had not “properly” translated them at the time. Officials quoted in one story refused to describe the content of those conversations.

On Sept. 11, 2002, Ramzi bin al-Shibh is captured by Pakistani police in a shootout in Karachi. He is currently in U.S. custody and, according to a U.S. Defense Department official, is “providing valuable information.” Bin al-Shibh is believed to have knowledge of Al Qaeda operations in Europe and Southeast Asia.

2004 Update: Khalid Shaikh Mohammed now in U.S. custody - see details.