Osama warned not long after 9/11 that soon many more planes would be falling out of the sky. Perhaps this is what he was referring to. From [1] AP, :

WASHINGTON (AP) -- President Bush gave new details Thursday about a foiled terror attack in 2002 in which plotters planned to use hijacked commercial airplanes to strike the West Coast.

Bush said the West Coast plot targeted the tallest building in Los Angeles, since renamed the US Bank Tower, and involved Khalid Sheik Mohammed, the mastermind of the September 11 attacks, who was captured in 2003.

Bush had referred to the plot before. In an address last October, Bush said the United States and its allies had foiled at least 10 serious plots by the al Qaeda terror network in the past four years, including plans for September 11-like attacks on both U.S. coasts.

The White House initially would not give details of the plots but later released a fact sheet with a brief, and vague, description of each.