Mueller told a congressional committee that the National Academy of Sciences would go over the scientific methods the FBI used to narrow its focus to U.S. Army scientist Bruce Ivins.

Mueller told the U.S. House of Representatives Judiciary Committee that there were open questions in the wake of Ivins' suicide in July, Fox News reported.

He said the academy would review the scientific findings because the validity of the techniques hadn't been tested in court.

Fox said one issue is the anonymous "Quantico letter" that warned police in the Virginia city that one of Ivins' colleagues at the Fort Detrick, Md., Army lab was a "potential terrorist" and was sent the same week anthrax powder showed up in the mail rooms at the New York Post and NBC.

The scientist in question was cleared of any involvement but has also stated he didn't think Ivins had sent the letter in a pre-emptive attempt to cover his tracks, Fox News said.