On September 11, 2001, terrorists linked to Osama bin Laden's al-Qaeda terror network hijacked four airliners. Two of the planes crashed into the World Trade Center towers in New York City, and a third into the Pentagon in Washington, DC. A fourth plane crashed in rural Pennsylvania en route to its suspected target, the U.S. Capitol building. The attacks and their dramatic demonstration of American vulnerability created an atmosphere of apprehension and uncertainty. Further attacks were anticipated, although there was a great deal of uncertainty as to when those attacks might occur and what form they might take. Against this backdrop, on 4 October 2001, health officials in Florida announced that Robert Stevens, a tabloid photo editor at American Media, Inc. (AMI), had been diagnosed with pulmonary anthrax the first such case in the United States in almost twenty-five years. Initially, the patient 's condition was attributed to a natural source. However, after two of the victim 's co-workers fell ill and anthrax spores were discovered throughout the building in which they worked, these initial assessments soon gave way to apprehension. Other cases began to appear at media outlets in New York City. These new cases revealed the possible source of the exposure: almost all of those infected in New York had come into direct contact with letters containing a mysterious powder. In mid-October, the crisis reached Washington, DC, when an anthrax-laden letter was opened in the office of Senator Tom Daschle (D-SD). Several workers at the postal facility that processed the letter fell ill with pulmonary anthrax. Congressional office buildings were evacuated and virtually all federal government mail delivery in the nation 's capital was halted as a result. An additional letter, addressed to Senator Patrick Leahy (D-VT), was found during a search of quarantined mail, bringing the total number of anthrax-laden letters sent to at least four.