American investigators were today looking into whether a fifth suicide hijacking was foiled when a flight from Boston to Los Angeles was cancelled.
The FBI is focusing on America Airlines Flight 43, which was cancelled at the last minute last Tuesday due to technical problems, and trying to determine if some of its passengers were terrorists.
The flight was due to take off from Boston's Logan airport 25 minutes after America Airlines flight 11, which was hijacked and flown into New York's World Trade Centre.
Today a source close to the investigation told the Chicago Tribune the FBI was "very interested" in the cancelled flight.
This is the first time firm details have emerged of a possible fifth flight being targeted by hijackers and comes after vice-president Mr Dick Cheney said the possibility of more hijacks could not be ruled out until the investigation was over.
The four hijacked flights were non-stop transcontinental flights, meaning they were laden with fuel, which made the fires which raged in the World Trade Centre and the Pentagon almost uncontrollable.
The FBI is also looking at the passenger lists of "several" other flights which were ordered to the ground in the aftermath of the hijackings.
And to help their investigation, the FBI has appealed for Arabic and Farsi speakers to help them interview the 49 people it has held as part of its investigation.
Most are being held under immigration regulations, but at least two are being held as "material witnesses", meaning FBI agents believe they have knowledge of the attacks which could be vital.
A total of 500 agents from 32 government departments are working round the clock at FBI headquarters overseeing the probe and thousands more are working on more than 40,000 possible leads in 12-hour shifts.
Authorities have taken databases from various government agencies, such as the immigration and naturalisation service and customs, into the FBI centre in New York to speed up their work.
And today it emerged many of bin Laden's family live in Boston and have been put under surveillance following the attacks.
A number of the family either live in Boston or have second homes there, while one is a Harvard-educated lawyer who practices law from the university town of Cambridge, Massachusetts.
All of bin Laden's American relatives have disavowed the suspected terrorist mastermind, but a man arrested as a material witness at John F Kennedy airport last Friday who gave an address in Boston linked to the hijackers, was also said to know one of the bin Ladens.
Bin Laden was one of 50 children born to a billionaire construction magnate who was a Yemeni immigrant to Saudi Arabia and made his fortune in the oil boom. The firm is now run by some of bin Laden's eldest brothers.
Many other members of the family have moved to America for university education and the family has given an endowment to Harvard to encourage the study of Islamic law, architecture and art.
The FBI and local police are combing credit card receipts from nightclubs and restaurants which younger members of the clan are thought to frequent in a bid to trace their activities and see if there are any links with the hijackers.