Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., one of the two senators targeted in the 2001 anthrax attacks, expressed his doubts on Wednesday about the FBI’s conclusion in the case.

During a hearing before the Judiciary Committee on Tuesday, FBI director Robert S. Mueller III said he was confident in the agency’s case, which declared scientists Bruce Ivins of US Army Medical Research Institute if Infectious Diseases at Fort Detrick in Frederick, Md. the lone attacker in the 2001 anthrax assaults that killed five people and sickened 17 others. Ivins committed suicide in July as federal investigators built their case against him.

It appears that a genetic match between the anthrax in the mailings and the toxin found in a flask in Ivins’ laboratory led investigators to the conclusion that he is the author of the attacks. Also, investigators “eliminated every other person” who might have had the skills to produce the material, Mueller said. He added that the National Academy of Sciences would review the scientific methodology used in the inquiry.

FBI “followed every lead to determine whether anybody else was involved, and we will continue to do so,” Mueller told lawmakers. He also added that even if the FBI closed the investigation on the anthrax attacks, this could be opened if anything pointing to other suspects came out.

But his testimony was highly criticized by Leahy who doesn’t believe that Ivin was the only one involved in the attacks.

“If he is the one who sent the letter, I do not believe in any way, shape or manner that he is the only person involved in this attack on Congress and the American people. I believe there are others involved, either as accessories before or accessories after the fact. I believe there are others who can be charged with murder,” Leahy told Mueller on Wednesday.

Sen. Arlen Specter (R-Pa.) also questioned the FBI’s conclusion saying there are “just so many questions unanswered. Did you personally review the evidence and come to a conclusion there was proof beyond a reasonable doubt?” Specter asked Mueller. He further suggested that Congress launch its own review of evidence.

Moreover, there are several lawmakers including Rep. Rush Holt, D-N.J., and Sen. Charles Grassley, R-Iowa, who are calling for an independent review of the FBI’s investigation. Grassley said Congress and the American people deserve a complete accounting of the FBI’s evidence because “there are many unanswered questions the FBI must address before the public can have confidence in the outcome of the case.”

Grassley further asked Mueller to explain the FBI’s position in Steven Hatfill’s case. Hatfill, another Army scientist, was wrongly accused of being involved in the attacks. Earlier this year, the Justice Department settled a $5.8 million lawsuit with Hatfill, who claimed that his privacy rights were violated by officials speaking with reporters about the case.