WASHINGTON  Senator Patrick J. Leahy, chairman of the Senate Judiciary Committee and a target of the anthrax letters of 2001, said Wednesday that he did not believe the F.B.I.’s contention that an Army scientist conducted the attacks alone.

At a hearing of his committee, Mr. Leahy told the F.B.I. director, Robert S. Mueller III, that even if the bureau was right about the involvement of the scientist, Bruce E. Ivins, who killed himself in July before ever being charged, he thought there were accomplices.

“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,” said Mr. Leahy, Democrat of Vermont.

“I believe there are others involved, either as accessories before or accessories after the fact,” he added. “I believe there are others who can be charged with murder.”