Among those unconvinced the FBI proved its case against Ivins, who committed suicide in July, is Sen. Arlen Specter, R-Pa., the ranking minority member of the U.S. Senate Judiciary Committee, The New York Times reported Sunday.
Specter told the newspaper: "My conclusion at this point is that it's very much an open matter. There are some very serious questions that have yet to be answered and need to be made public."
The Times said that in interviews with two dozen bioterrorism experts, veteran investigators and members of Congress, many doubts were expressed that FBI Director Robert Mueller had the right man when he named Ivins as the perpetrator of the 2001 mailings of anthrax-laced letters.
Critics say the FBI hasn't sufficiently explained why it didn't focus on Ivins until last year, nearly six years after the attacks, or why it ruled out nearly 100 other people who had access to the anthrax at Ivin's U.S. Army lab.
Notable deaths of 2014 [PHOTOS]