The FBI yesterday embarked on what is certain to become the biggest investigation in its history, as agents followed early leads which led to Maine, Florida, Canada and to a mislaid bag containing a Koran and a video on how to fly a commercial plane.
In Boston, heavily-armed agents and police swarmed into a central hotel shortly before 1 p.m. local time. An armoured vehicle was parked outside and officers in riot gear cordoned off the site and made several arrests. A bomb squad and ambulances were also at the scene. A spokeswoman for the justice department, Mindy Tucker, said investigators were following up 700 tip-offs sent to the FBI website. She refused to speculate on the agency's initial suspicions.
However, Senator Orrin Hatch said US intelligence had intercepted communications between supporters of Mr Osama bin Laden, discussing the attacks on the World Trade Centre and the Pentagon. "They have an intercept of some information that included people associated with Bin Laden who acknowledged a couple of targets were hit," he said.
FBI agents executed search warrants in Florida and interviewed a couple who housed two men believed to be involved in the hijackings. The bureau served search warrants on major internet service providers, seeking information about an e-mail address that may be connected to the attacks. AOL, the largest US provider, said it would comply quickly.
Almost every FBI agent in the country was thrown into the search for clues. They combed the crash sites, pored over passenger lists, scanned hours of airport surveillance videos in the hope of identifying the hijackers and clues on how they smuggled their weapons through security. They also checked abandoned cars and conducted word-by-word analyses of last-minute calls from the hijacked airliners.
The probe initially focused on the point of departure of each of the four doomed flights. Hundreds of agents and police officers descended on Boston's Logan airport, the departure point for the two airliners which destroyed the World Trade Centre's twin towers. Investigators also flooded Washington's Dulles airport, where the hijackers boarded an American Airlines Boeing-757 and flew it into the side of the Pentagon.
The search also extended to Newark airport and one in Portland, Maine. At each airport, the FBI examined passenger lists, cross-checking names against lists of suspected terrorist sympathisers. In Logan, the examination produced five names of suspected hijackers, two of them brothers travelling on United Arab Emirates passports. Agents also carried out a comprehensive check on vehicles in the airport car parks - a search which yielded an Arabic-language flight training manual.
FBI agents were also interviewing a couple in the west Florida town of Venice, who said they hosted two men in their house in July 2000. They said the men were learning to pilot small aircraft at a local flight training school.