The FBI reveals the identities of the 19 alleged hijackers and launches the biggest investigation in its history. US Attorney-General John Ashcroft says that all roads in the investigation lead to Saudi-born dissident Osama Bin Laden and his al-Qaeda organisation. In the initial days after the attacks, the FBI says it is following up 50,000 leads. More than 4,000 people are put on the case.
Congress approves the use of force in response to the attacks and releases $40bn emergency spending.