House/Senate Intelligence Committee: Intelligence agencies were aware
According to the Wall Street Journal (subscription required):
Intelligence agencies were aware of numerous threats that terrorists might use airplanes as weapons against the U.S., despite government claims following Sept. 11 that the World Trade Center and Pentagon attacks came like bolts from the blue.
...For example, the report detailed examples of intelligence showing Osama bin Laden intended to carry out a strike inside U.S. borders, including an August 1998 report that "a group of unidentified Arabs planned to fly an explosive-laden plane from a foreign country into the World Trade Center." The Federal Bureau of Investigation relayed the information to the Federal Aviation Administration, which deemed the plot unlikely, the report said.