Saudi Arabia announced that al Qaeda had planned to deal a serious blow within the United States in 2003:
The official told the Associated Press that the 2003 plot involved militants who planned to hijack a plane and blow it up over a densely populated US city.
The militants planned to arrive in the US illicitly via a third country – such as Canada or Mexico – so they could avoid applying for a US visa.
According the Saudis, they have prevented 160 total terrorist attacks. This scenario seems plausible, but one must remember that a wing of the Saudi Royal Family was responsible for fostering al Qaeda during the 1990s, just as the Pakistani ISI fostered the Taliban. Some in the Saudi Royal Family or intelligence services may still have al Qaeda sympathies.