According to a declassified CIA intelligence report from December 4, 1998: “IG [terrorist group Gama’at al-Islamiyya] leader Islambuli in late September was planning to hijack a US airliner during the ‘next couple of weeks’ to free [World Trade Center bombing conspirator Shaykh Umar] ‘Abd al-Rahman and the other [co-conspirator] prisoners… The same source late last month said that Bin Ladin might implement plans to hijack US aircraft before the beginning of Ramadan on 20 December and that two members of the operational team had evaded security checks during a recent trial run at an unidentified New York airport [redacted].”
– CIA, “Bin Ladin Preparing to Hijack US Aircraft and Other Attacks,” (CIA Intelligence Report), The Central Intelligence Agency’s 9/11 File:, The National Security Archive, Dec. 4, 1998