The letters appeared to have been mailed from Princeton, NJ and a public mailbox near the Princeton University campus was found to be contaminated with anthrax spores. Letters mailed to Democratic Senators in October 2001 contained a similar threatening note referencing the September 11th attacks. In all, 17 people were sickened and five were killed due to anthrax exposure. Victims included assistants who opened or handled the letters, postal workers who sorted the letters, and a woman from the Bronx whose exposure could never be determined.

Although the letters were written to appear as if they were from Muslim extremists, an FBI investigation identified the perpetrator as an American microbiologist, vaccinologist, and senior biodefense researcher at a government facility in Maryland. Bruce Edwards Ivins, a 62-year-old white male, became the focus of the investigation seven years after the incidents, after which he started to show signs of strain and was involuntarily committed to a psychiatric hospital. During therapy sessions, he discussed the anthrax mailings in a suggestive way, but did not confess to being involved. Ivins died by suicide on July 29, 2008 after learning that he had been identified, and the FBI formally closed the case in 2010, identifying Ivins as the sole perpetrator of the letters and detailing how he falsified evidence and attempted to frame co-workers at the governmental research lab to throw investigators off the case.