FBI Director Robert Mueller told Congress on Tuesday that the bureau is submitting its scientific findings in the anthrax probe to an independent review, as new evidence appears to continue to unravel the law enforcement investigation.

The National Academy of Sciences will review the "science" used by the FBI to link anthrax in letters sent to news organizations and Capitol Hill offices in September 2001 to a flask in the laboratory of an Army scientist who committed suicide in July

The FBI probe concluded this summer that Bruce Ivins, the late scientist at Ft. Detrick accused of mailing the letters, was the sole and primary suspect.

The case is going to the NAS in part because the methods employed by the FBI were specifically developed for the investigation and have never been tested in court for its reliability. In fact, it is so "new and cutting edge" it has never been used in a case of such magnitude.

Mueller told the House Judiciary Committee about the decision as the bureau faced lingering questions, not only from some scientists at Ft. Detrick who say they are not convinced of Ivins' guilt but also from some lawmakers who wondered, if the FBI was so convinced about Ivins' guilt, why was he never charged?

Mueller is testifying to the House and Senate on both Tuesday and Wednesday as lawmakers grill Mueller about new information that raises more questions about the FBI's conclusions.

"My conclusion, at this point, is that it's very much an open matter," Sen. Arlen Specter, R-Pa., Senate Judiciary Committee co-chairman wrote in a letter to Mueller less than two weeks ago.

House Judiciary Committee Chaiurman John Conyers complained Tuesday that his committee had submitted more than a half dozen questions to be answered, including on anthrax, but the committee was not getting answers.

Among the sources for the questions raised is a mysterious letter, sent in the fall of 2001, warning of a bioterror attack. At the FBI, the mailing is known as the "Quantico letter" because it was sent to the police in the Virginia town that is home to several military and government agency facilities.

The letter was sent the same week anthrax was mailed to The New York Post and NBC. In the letter, first obtained by FOX News, an Army scientist at Ft. Detrick in Maryland is accused of plotting against the U.S. government:

"This guy is a potential terrorist," the letter reads. "This guy has access to many dangerous biological poisons. ... Please talk to him to make certain that he is not involved in further terrorist activity."

The anonymous letter accused Dr. Ayaad Assaad of also being a religious fanatic. Assaad was interviewed by the FBI and cleared of any role in the anthrax case. But the central question that remains is whether the Quantico letter was part of an effort to frame Assaad two weeks before anthrax killed the first of five Americans.

"It was deceptive in one way and the other way it would fit to accuse an Arab-American after the 9/11 attack of committing this crime," Assaad said.

Publicly, the FBI and the U.S. attorney's office insist that Ivins was the sole suspect in the anthrax case. When asked by FOX News about the Quantico letter, officials called it irrelevant to the case.

"Not aware of any connection to my knowledge, there's no evidence linking the two," said Jeffrey Taylor, U.S. attorney for the District of Columbia.

While FBI officials now say the Quantico letter has no connection to the anthrax case, in July of 2003, the bureau was telling a different story. In denying a Freedom of Information Act request, the FBI claimed the letter's release "could reasonably be expected to disclose the identities of confidential sources."

Assaad, who worked alongside Ivins for 18 years, said his late colleague is unlikely to be a source of the letter.

"Bruce Ivins is an honorable man. We're good friends. That is not the writing of Bruce Ivins," Assaad said.

After Ivins’ suicide, the FBI alleged that the scientist stole the anthrax powder from the base and mailed it. But some scientists at Ft. Detrick, a major Army bioweapons research facility, still question whether Ivins could have been solely responsible for the attack.

FOX News has learned that an independent analysis done for the FBI shortly after the Sept. 11, 2001, terror attacks concluded that the most likely writer of the Quantico letter was a female scientist at Ft. Detrick.

The similarities between the typed Quantico letter and handwritten anthrax letters are also striking beyond the obvious connection to Ft. Detrick.

Both warn of biological attacks in fall of 2001. Both express hatred for Israel. Both begin with the word "This," which investigators say is a highly unusual stylistic quality.

The letters also contain prominent spelling mistakes. In the Quantico letter, the spelling of the Jewish state is "Isreal." In the anthrax letters, Penicillin is spelled "Penacilin."

One source says the spelling mistakes were an effort to obscure the writer's identity. But the addresses on the envelopes to both the Quantico letter as well as the anthrax letters do not have a comma between the city and state, another potential clue.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.