Enlarge By Mandel Ngan, AFP/Getty Images Sen. Patrick Leahy, left, welcomes FBI chief Robert Mueller on Wednesday at the Judiciary panel hearing. WASHINGTON  Senate leaders on Wednesday expressed serious doubts about the FBI's assertion that Army scientist Bruce Ivins was the lone attacker in the 2001 anthrax assaults that killed five people and sickened 17 others. A day after FBI Director Robert Mueller said he was confident in the agency's case, Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., one of the two senators targeted in the attacks, said he believes there are "others who could be charged with murder." "I do not believe in any way, shape or manner that he was the only one involved," Leahy told Mueller at a committee hearing. Ivins, an anthrax scientist at the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick in Frederick, Md., committed suicide in July as federal investigators built their case against him. Paul Kemp, Ivins' attorney, says the scientist was innocent. Mueller defended the case Tuesday in testimony before the House Judiciary Committee, saying that investigators had made a "clear identification," tracing the deadly anthrax contained in a series of letters to the substance in a vial that Ivins controlled at the Maryland laboratory. Sen. Arlen Specter of Pennsylvania, the committee's top Republican, said he also has "grave doubts" about Ivins' guilt and suggested that Congress launch its own review of the evidence. At one point, Specter asked Mueller whether he had personally reviewed the evidence and was convinced "beyond a reasonable doubt" of Ivins' guilt. "Yes," Mueller said, adding that the National Academy of Sciences will review the scientific methodology used in the inquiry. The director's repeated assurances, however, did not settle the matter. "It was my impression that they felt their scientists were authoritative and yet they are still seeking affirmation of their theories even though they have told us the 'case is closed,' " Kemp said. "If further study is needed, then nothing is finally resolved." "Dozens and dozens of serious questions remain unanswered," said Sen. Chuck Grassley, R-Iowa, said. "If this case truly has been solved, then there has to be a process capable of ensuring that the FBI got it right." Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more