A senior Republican senator says it would take a powerful grassroots movement or startling new evidence to reopen the Justice Department's investigation that branded a now-deceased Army researcher as the anthrax mailer who killed five people a decade ago.

Iowa Sen. Charles Grassley, the ranking Republican on the Senate Judiciary Committee, and others on Capitol Hill who've been skeptical of the case against the late Bruce Ivins said adamant opposition from the FBI and Justice Department is likely to block further inquiry into the case.

Even if he were the committee chairman, Grassley said, "I would question my capability of raising enough heat (to reopen the case) when you're up against the FBI. And I've been up against the FBI."

Members of Congress commented after PBS' Frontline, McClatchy and ProPublica, in a joint investigation, disclosed evidence that's at odds with some of the science and circumstantial evidence behind the government's conclusion.

Rep. Rush Holt, D-N.J., who has criticized the FBI investigation as "botched" and from whose district the deadly letters were mailed, said he may try for a third time to win support for legislation creating a special commission to investigate the attacks.

"There are so many reasons to want to get to the bottom of it," Holt said in an interview. "I hate to think of what lines of investigation have been shut off."

Holt, who is a physicist, traced some of the resistance to the fact that Congress "has never felt comfortable dealing with scientific issues," as well as to the public wishing to forget "an unpleasant occurrence."

Rep. Jerrold Nadler, a New York Democrat who has supported Holt's bill, "is frustrated that the FBI has failed to answer all of his questions," said his spokesman, Ilan Kayatsky. However, Kayatsky said, "it does seem unlikely at this time that they will reopen their investigation."

Ivins, a mentally troubled father of two who worked for 27 years at the U.S. Army Medical Institute of Infectious Diseases (USAMRIID) at Ft. Detrick, Md., committed suicide on July 29, 2008, not long after learning that federal prosecutors were preparing to seek his indictment on five capital murder charges.

Last year, prosecutors closed the FBI's eight-year, $100 million investigation and formally branded him the killer in a 93-page report that laid out extensive evidence against him.

Nearly all of the evidence was circumstantial, however, and PBS' Frontline, McClatchy and ProPublica, in a one-hour documentary and a three-partnewspaperseries, disclosed evidence challenging prosecutors' assertions.

Among the evidence the three news organizations scrutinized:

• FBI claims that Ivins worked unusually late hours in a "hot suite," a secure bio-containment lab at Ft. Detrick, in the weeks before the letter attacks. Records show that Ivins had worked similar evening hours in other USAMRIID facilities in the preceding months.
• Assertions that Ivins tried to mislead investigators in April 2002, by manipulating anthrax samples from a laboratory flask he submitted for FBI testing. At issue was whether Ivins was trying to keep investigators from discovering that spores in the flask contained the same genetic variants as those in the anthrax contained in the letters. But while the April samples tested negative for the variants, Ivins gave three other samples to the FBI or fellow researchers between 2002 and 2004 and, ultimately, the bureau recorded positive results in tests of all three, FBI and Army records show.
• Claims that Ivins was motivated to create fear about anthrax because the government's anthrax vaccine program was under heavy fire. The existing program was under fire, and Ivins helped to address problems, but his job was to develop a second generation vaccine that at the time had full funding.
• Assertions that science showed that Ivins' flask was "effectively the murder weapon." A panel of the National Academy of Sciences and two scientists who worked on the FBI investigation described holes in that and other laboratory conclusions.

The lead prosecutor in the case, Rachel Lieber, dismissed these and other anomalies, saying that the "big picture" consisting of a vast "mosaic of evidence" presented a powerful case that Ivins' mental problems drove him to commit the crimes.

Former FBI agent Brad Garrett, a profiler who advised agents in the investigation periodically before retiring in 2006, said that Ivins fits the mold of the suspect the bureau was hunting, because he was "a really super angry guy ... and dangerous on some levels. He clearly had grudges with people."

However, no direct evidence has confirmed that Ivins held grudges against the media outlets to which the letters were sent.

To reopen the case, Garrett said, would take "something fairly compelling ... somebody comes forward [or] there's a new piece of evidence that links it to somebody else."

Holt and Rep. Roscoe Bartlett, R-Md., whose district includes Ft. Detrick, tried to push through an amendment to a spending bill last year requiring the inspector general for the intelligence community to investigate whether all relevant foreign intelligence had been passed to FBI investigators. The measure was torpedoed when the Office of Management and Budget objected, calling it "duplicative" and expressing concern about Congress directing an inspector general "to replicate a criminal investigation."

Last May, McClatchy disclosed that the FBI had never explained tests showing the presence of unusually high levels of silicon and tin in the letters sent to the New York Post and to Democratic Sen. Patrick Leahy of Vermont. That renewed suspicions -- denied by the FBI -- that the perpetrator used a chemical additive to keep the spores from clumping so they'd be more easily inhaled.

Another issue is the FBI's method for collecting anthrax samples from U.S. and foreign labs to narrow the suspect list. Because the samples were subpoenaed and couldn't be seized for multiple reasons, critics have said their submission amounted to an honor system in which the killer would have no incentive to participate.

Further, a still-confidential 2002 review of security at USAMRIID by a seven-member team from the Sandia National Laboratories found that "the culture at USAMRIID does not reflect the same indisputable commitment to security as it does to research."

The "diversion of small quantities" of deadly pathogens can be significant, noted the report, a copy of which was obtained by McClatchy, ProPublica and Frontline. That's presumably because they can be used as seed material to grow large quantities of germs for an attack. The problem is heightened, it said, because germs "cannot be reliably detected," underscoring the importance of an alert and cooperative research staff.