A year before the U.S. embassy bombings in East Africa, the FBI and the Kenya police raided the house of Wadih El Hage in Nairobi, Kenya in August of 1997 and found a very disturbing letter on his computer hard drive. The letter, which the FBI believes was written by Fazul Abdullah Mohammed, AKA Haroun Fazul, outlines the presence of a bin Laden terrorist "cell" in Nairobi and clearly states the member's knowledge of bin Laden's call to kill Americans.

If United States intelligence officials knew about suspect activity in Kenya a year before the bombings, could they have prevented the tragedy? Intelligence officials complain that they receive so many threats that it is difficult to predict which are true - although they do follow up on all of them.

The FRONTLINE and The New York Times investigation into Osama bin Laden uncovered warnings that were received by U.S. officials about bin Laden's activities in Africa in the year prior to the Embassy bombings.

As early as 1994, United States intelligence officials were receiving worrisome reports about Osama bin Laden, according to FRONTLINE's interview with former CIA and State Department official Larry Johnson. Then in early 1995, Ramzi Yousef, mastermind of the World Trade Center bombing, was captured in Pakistan and was linked to Osama bin Laden.

By 1996, the United States Justice Department quietly convened a grand jury in New York to investigate Osama bin Laden, and FBI investigators began building a case against him. The CIA was simultaneously gathering intelligence about bin Laden and his associates. Some of this evidence led the investigators to Kenya. In Kenya, there were two concrete warnings about an embassy attack during 1997.

According to intelligence sources, the CIA and FBI kept U.S. Ambassador to Kenya Prudence Bushnell and her staff fully informed about the warnings and the possible threat. However, some embassy officials dispute this, saying they were not kept in the loop about the seriousness of the threat. Throughout this period, Bushnell was pleading with the State Department to provide a new building or seriously upgrade security at the old building. But the State Department did not consider Nairobi a high-risk location and ignored Bushnell's pleas.

Summer, 1997 -- Wadih El Hage, who worked as a secretary for bin Laden in the Sudan, was found to be living in Nairobi, Kenya. The FBI, CIA and Kenyan police cooperated on the August, 1997 raid on Wadih El Hage's home. Under the pretext of searching for stolen goods, they entered the house and took Mr. El Hage's computer, according to El Hage's mother-in-law, who was present during the raid. On the hard drive of that computer, investigators found a chilling letter, which they now believe was written by El Hage's houseguest, Fazul Abdullah Mohammed or Haroun Fazul. Fazul was also linked to bin Laden in the Sudan and had been living with the El Hage family in Kenya.

The letter clearly outlines the presence of a clandestine "cell" in Nairobi, operating under the instructions of "the Haj" or Osama bin Laden. Just after the raid, the investigators interviewed Wadih El Hage in Nairobi. According to El Hage's mother-in-law, the FBI repeatedly told both El Hage and his wife to leave the country. The agents insinuated that the family was in danger should they remain in the country. El Hage took them seriously and returned to the United States with his family in September of 1997.

It is unclear whether the investigators knew about Haroun Fazul at this time, or understood that he had been the author of the letter. Fazul somehow escaped their notice and continued his activities in Nairobi. He would later be declared the mastermind of the Nairobi bombing. In fact, on the day of the actual bombing, he drove the lead truck that led the bomb to the United States Embassy.

November, 1997 - Near the end of 1997, an Egyptian named Mustafa Mahmoud Said Ahmed walked into the United States Embassy in Nairobi and told a CIA agent that he knew of a plan to detonate a truck bomb in the parking garage of the embassy. He was interrogated by Kenyan police and admitted that he had taken part in surveillance of the embassy, including taking photos.

For unknown reasons, the CIA did not take Mr. Ahmed's statements seriously and he was deported. The CIA issued two intelligence reports on the warning, but called into question the credibility of Mr. Ahmed.

Mr. Ahmed was arrested in Tanzania just after the 1998 summer bombings and has been charged in conjunction with the Dar Es Salaam bombing attack.