WASHINGTON (Reuters) - A senator targeted in the deadly 2001 anthrax attacks said on Wednesday he is convinced that the man believed to have carried it out did not act alone.

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Senate Judiciary Committee Chairman Patrick Leahy, a Vermont Democrat, voiced doubts to FBI Director Robert Mueller at a hearing in which other lawmakers called for a stepped-up review of the FBI’s case against U.S. Army scientist Bruce Ivins.

Last month the FBI and Justice Department said Ivins, an anthrax expert who killed himself in July, was solely responsible for the mailing of anthrax-laced envelopes to politicians and media organizations shortly after the September 11 attacks.

The mailings killed five people and sickened 17. One of the letters was addressed to Leahy, but it was misdirected to another building and he was unharmed.

“If he (Ivins) was 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 or the American people,” Leahy told Mueller. “I believe there are others out there. I believe there are others who can be charged with murder.”

Leahy did not explain his views, and a spokeswoman did not immediately respond to a request for elaboration.

FBI officials have said the investigation is over and would be formally closed soon. But Mueller told Leahy the bureau would remain open to considering new evidence that pointed to additional suspects.

“We have looked at every lead and followed every lead to determine if anyone else was involved and will continue to do so,” Mueller said.

Ivins’ death and the FBI’s disclosure that new DNA analysis techniques helped crack the case have sparked questions over the reliability of the evidence against Ivins. His lawyer says he was innocent.

Mueller on Tuesday announced that the FBI had asked the National Academy of Sciences to review the scientific evidence, but some lawmakers said it was not enough.

Republican Sen. Charles Grassley of Iowa sought a congressional probe, and Republican Sen. Arlen Specter of Pennsylvania urged Mueller to let Congress name some members of the review panel.

“There has to be some alternative process capable of ensuring, in a way that a trial could have, that the FBI got it right,” Grassley said.

FBI investigators mistakenly focused for years on another scientist, Steven Hatfill. Hatfill was never charged and the government agreed in June to pay him $5.85 million to settle a lawsuit. Hatfill was seen in the audience for Wednesday’s hearing but he did not address the lawmakers.