September-October 2001: Letters containing anthrax were mailed to news outlets and to Democratic senators Tom Daschle and Patrick Leahy. More than a dozen people got sick; several died.
2002: Media outlets and law enforcement named a "person of interest" in the attacks, bio-weapons expert Steven J. Hatfill. He later sued the government and The New York Times for ruining his reputation. The federal government paid $5.8 million to settle his suit against the Justice Department.
April 2004: The FBI seizes samples from Bruce E. Ivins' lab. He worked with anthrax at the United States Army Medical Research Institute of Infectious Diseases in Maryland.
2008: Ivins commits suicide after being told he would likely be charged in connection with the attacks. The FBI officially named Ivins as the person responsible for the attacks.
Feb. 19, 2010: The FBI officially closed the case.