FBI Is Casting A Wider Net in Anthrax Attacks

Moreover, scientists say, the particular strain of anthrax used in the attacks has turned to out to be a less significant clue than first believed. The highly virulent Ames strain was first isolated in the United States and was the basis for the anthrax weapons formerly created by the United States. The use of the Ames strain in the 2001 attack was initially seen as a strong clue linking the terrorist to the U.S. biodefense network.

But the more the FBI investigated, the more ubiquitous the Ames strain seemed, appearing in labs around the world including nations of the former Soviet Union.

"Ames was available in the Soviet Union," said former Soviet bioweapons scientist Sergei Popov, now a biodefense expert at George Mason University. "It could have come from anywhere in the world."

Many law enforcement officials believe that ever-improving technology eventually could lead to a break in the case. Ongoing tests could lead authorities to the lab where the anthrax originated -- something authorities have said for years could help close the case.

More traditional tactics are still being used: The FBI has conducted 9,100 interviews and issued 6,000 subpoenas in one of the most exhaustive and expensive investigations in the bureau's history. Authorities say investigators continue to have a number of specific individuals in their sights, describing the suspect list as "fluid."

One prevailing theory among investigators is that the attacks came from within the United States rather than from an overseas terrorist organization.

However, a law enforcement official said, "we have not closed the door on any possibilities. There's a discrete number of individuals who continue to be investigated, both internationally and domestically."

Over the years, officials have publicly identified only one "person of interest," and that was more than four years ago. Steven J. Hatfill, a former Army scientist, has denied wrongdoing and has never been charged. He is suing the Justice Department, alleging that officials leaked false information about him that caused great harm.

Law enforcement officials won't talk about Hatfill.

Homeland Security Secretary Michael Chertoff, in a meeting this month with Washington Post reporters and editors, would not say whether any single individual continues to draw special attention as a "person of interest."

"I'm not telling you that right now the bureau is focused on someone or not focused on someone," Chertoff said. "There are in my experience a lot of instances where we might know or have a good reason to believe who committed a criminal act, but we may not be able to prove it. So when you say something is not solved, you should not assume from the fact that there is no criminal prosecution we don't have a good idea of what we think happened."

Persichini, of the FBI's Washington office, acknowledged frustrations but said that "no one in the FBI has for a moment stopped thinking about the innocent victims of these attacks, nor has the effort to solve this case in any way been slowed.

"While not well known to the public, the scientific advances gained from this investigation are unprecedented and have greatly strengthened the government's ability to prepare for -- and prevent -- biological attacks in the future," Persichini said.

Nonetheless, failure to solve the mystery has bred public skepticism.

"If the FBI's investigation has become a cold case, then it's time for [FBI Director Robert S. Mueller III] to acknowledge that and take steps to deal with it," said Sen. Charles E. Grassley (R-Iowa), a frequent critic of the FBI. "I'm concerned that the FBI may have spent too much time focusing [on] one theory of what happened and too little effort on the other possibilities."

Staff writer Spencer S. Hsu and staff researcher Meg Smith contributed to this report.