The month before the Sept. 11 hijacking attacks, the CIA received information suggesting Osama bin Laden was increasingly determined to strike on U.S. soil. In the days since, the FBI has linked the hijackers to bin Laden's network through phone intercepts, money transfers and training camps.
U.S. officials, speaking on condition of anonymity, described to The Associated Press a trail of evidence they believe points to bin Laden's involvement in the attacks by 19 terrorists who crashed four airliners.
Among the key pieces of evidence, the officials said, is a series of money transfers between hijacking ring leader Mohamed Atta and a Middle Eastern man named Mustafa Ahmed. The last occurred a few days before the hijackings when Atta wired money back to Ahmed in the United Arab Emirates, the officials said.
The FBI believes the transfers may provide a clear link to bin Laden. Agents are investigating whether Ahmed was an alias for a man named Shayk Saiid that U.S. authorities long have believed helped run bin Laden's finances, the officials said.
In documents sent to banks seeking to freeze terrorist assets, the government has used Saiid's and Ahmed's names interchangeably, records show.
Ahmed is believed to have left the United Arab Emirates on Sept. 11 for Pakistan and is a major focus of the FBI's global manhunt.
Other evidence includes "general but vague" information the CIA developed in August that heightened concerns that bin Laden was urging his followers to strike on U.S. soil after several attacks overseas in the 1990s.
The information indicated bin Laden and his supporters "were trying to bring the fight to America" but details were lacking, a U.S. official told The Associated Press.
"There was something specific in early August that said to us that he was determined in striking on U.S. soil," the official said, speaking only on condition of anonymity. "But there was nothing about who, when, where or how."
The information prompted the CIA to issue a renewed warning that U.S. interests overseas and at home should be vigilant, the officials said.
But U.S. intelligence officials, along with congressional officials who have been briefed on the evidence and cooperating foreign intelligence agencies all told the AP that the CIA did not possess any information that identified a specific plot of the magnitude that struck the World Trade Center and the Pentagon.
"We've got plenty of areas we can improve, but I don't want anybody to get the idea that that was a great intelligence failure," House Intelligence Committee Chairman Porter Goss, R-Fla., said this week. "If it's anything, it's probably, 'Who would have expected an atrocity of that magnitude?'"
The CIA's August warning is being viewed as a piece in the puzzle of evidence. Another piece, officials said, involves a meeting two other hijackers, Khalid Almihdhar and Nawaf Alhazmi, had with a bin Laden associate in January 2000 in Malaysia.
The meeting was observed and recorded at the time, but its significance was not apparent until just a few weeks before the attacks, officials said. By that time, the two men already were somewhere in the United States.
The Malaysia meeting took on new significance when U.S. investigators developed evidence in Yemen this year that the man the two hijackers met with in 2000 was involved in the planning of the USS Cole bombing, the officials said.
Other evidence comes in the form of intercepted communications. German authorities have confirmed they intercepted a conversation of bin Laden supporters celebrating the suicide hijackings.
Another intercept, confirmed by Sen. Orrin Hatch, R-Utah, earlier in the investigation, detected a bin Laden sympathizer indicating that intended targets had been hit on Sept. 11.
Information gathered by U.S. and foreign intelligence services and law enforcement, officials said, also indicates four of the hijackers trained at Afghan camps tied to bin Laden's network.
Officials said the four included Wail Alshehri. Uncorroborated intelligence indicates he received several months of training last year in hand-to-hand combat, bomb-making and poison-mixing at Al Farooq camp in Khandhar, Afghanistan, officials said.
The others linked to Afghan camps are Waleed Alshehri, believed to be a brother of Wail Alshehri, Hamza Alghamdi and Nawaq Alhamzi, the officials said.