Further, when Bush attended the G8 Conference Genoa (summer of 2001), somebody thought to provide anti-aircraft weapons around the buildings. Why, there had been intelligence (airplanes used as weapons) that this sort of attack might happened.
http://montreal.indymedia.org/front.php3?article_id=4049&group=webcast
Officials said Italy took special precautions during the July Group of Eight summit in Genoa after Rome and Washington were warned of terrorist plans to crash an airliner into the summit site, the Los Angeles Times reports. The Italians closed the airspace over Genoa and positioned anti-aircraft guns at the city's airport, but, according to a U.S. official, U.S. counterintelligence considered the warning "unsubstantiated" (Los Angeles Times, Sept. 27).