In the fall of 2001 envelopes carrying deadly Anthrax were delivered to U.S. Senate offices, network news divisions and a tabloid newspaper. Five people were killed, many more infected and the nation was terrorized. Seven years later, after mistakenly pursuing one suspect, the most expensive and complex investigation ever undertaken by the FBI ended when they identified Army scientist Dr. Bruce Ivins as the sole perpetrator of the attacks -- after Ivins had taken his own life. Now, new questions are being raised about the FBI's investigative methods and whether Ivins really did it. FRONTLINE, in a co-production with ProPublica and McClatchy Newspapers, takes a hard look at the FBI's investigation of the country's most notorious act of bioterrorism.