In October 2001, letters containing virulent anthrax bacteria were sent through the U.S. mail. Between October 4 and November 20, 2001, 22 individuals developed anthrax and 5 of these cases were fatal. The FBI worked with other federal agencies to conduct scientific analyses of the spore powders in the letters as well as environmental, clinical, and laboratory samples to determine their source. In 2008, the FBI asked the National Academy of Sciences to conduct an independent review of the scientific approaches used during its investigation of the anthrax mailings.