Or Khalid could have read Clancy's follow-on book Executive Orders, at the beginning of which Captain Sato lands on Congress, catapulting Jack Ryan into the White House, ready to cope with a bio attack, among other things.
Or Khalid might simply have read the news. On 7 April 1994, a very nonfictional and profoundly disgruntled Auburn Calloway, just fired from Federal Express, came damn close to doing a 9/11 on his former employer's Memphis hub.
Also, Sept 12, 1994, a small plane crashed into the white house south lawn. I think anti-aircraft guns were installed afterwards.