The main suspect, Bruce Ivins, died in an apparent suicide in 2008, his motive unknown .

ECONOMIST: A history of parcel bombs

The FBI eventually concluded that Bruce Ivins, a microbiologist working at an Army lab in Maryland who committed suicide in 2008, had sent the letters.

WSJ: Ricin Also Suspected in Letter to Obama

In an ironic twist, the likely perpetrator of the attacks, Bruce Ivins, was allegedly motivated by his interest in reviving the anthrax vaccine program.

FORBES: The anthrax vaccine boondoggle

Tom Daschle, who was Senate majority leader at the time, said he is suspicious of the case against researcher Bruce Ivins because of the government "bungling" of Steven Hatfill's case.

CNN: Intended anthrax target has doubts about probe