The person responsible for the 2001 anthrax attacks was Bruce Ivins.
Background
The 2001 anthrax attacks were a series of bioterrorism attacks in the United States that occurred in the weeks following the September 11 attacks. Letters containing anthrax were mailed to several news media offices and two Democratic US senators, resulting in 22 reported cases of anthrax infection, including five deaths.
Investigation
The FBI began an extensive investigation into the origin of the anthrax. In February 2002, the FBI identified the Ames strain of anthrax as the strain used in the attacks, and identified the US Army Medical Research Institute of Infectious Diseases (USAMRIID) as the source of the strain.
Suspect
The FBI identified Bruce Ivins, a research scientist employed by USAMRIID, as a suspect in the attacks. After an extensive investigation, the FBI concluded that Ivins was solely responsible for the attacks. On July 29, 2008, Ivins committed suicide before he could be indicted and brought to trial.