But soon the FBI began keying on a so-called "person of interest" – Steven Hatfill – and launched one of the largest criminal investigations in its history.
As correspondent Lesley Stahl reports, the FBI has been going after this guy for five years, and yet he has got them in court: Hatfill has sued the FBI and Department of Justice for what he claims has been a campaign of leaking lies and distortions about him to the press.
Through the lawsuit, Hatfill's lawyer has not only obtained boxfuls of internal government documents, but he has also deposed nearly every major law enforcement official involved in the case. It is the latest twist in the FBI's yet unsolved investigation of the anthrax murders.
A number of anthrax letters began appearing in the mail between late September and October, 2001. The letters were sent to news outlets – Dan Rather and Tom Brokaw got them. As did two U.S. senators. And two postal workers, who handled the poisonous envelopes, died.
Steven Hatfill, a medical doctor and an expert on viruses, was outed in a drumbeat of news reports that included aerial shots of the FBI seizing property from his apartment, including his trash.
And then-Attorney General John Ashcroft confirmed on television that Hatfill was a "person of interest."
But instead of the FBI nailing Hatfill, he filed his lawsuit claiming that with their leaks, the FBI and Justice Department had violated his presumption of innocence and destroyed his reputation.
"I object to an investigation characterized, as this one has been, by outrageous official statements, calculated leaks to the media, and causing a feeding frenzy operating to my great prejudice," he said in August, 2002.
In the lawsuit, Hatfill is turning the tables on the FBI: the hunted is dragging the hunters into court. Top officials were deposed on videotape, like John Ashcroft, who was less than forthcoming.
"Is it appropriate for Department of Justice officials to suggest that Dr. Hatfill fits a behavioral profile of the anthrax killer?" Ashcroft was asked.
"I don't know," the attorney general replied.
"You don't know whether it was appropriate or inappropriate to disclose that kind of information?" he was asked.
"I don't know," Ashcroft responded.
Asked if he thought it was fair to Dr. Hatfill, Ashcroft said, "I don't know."
John Ashcroft answered "I don't know" to 85 questions in the four and a half hour deposition.
Hatfill came on the radar screen in the first place because he seemed to fit the FBI profile as an American scientist who had worked at a U.S. Army laboratory where the strain of anthrax used in the attacks was stored.
There were other – quote – "curiosities." For instance, he commissioned a study in 1999 of how emergency personnel should respond in the event of an anthrax mailing. He wrote a novel fictionalizing a bio-terrorist attack in Washington.
And there's an open question about how similar his handwriting is to that on the anthrax envelopes. But in his deposition, Richard Lambert, who oversaw the FBI investigation, said there were other people on the radar screen.
"There were 20 to 30 other people who were also likewise identified as 'persons of interest' in the investigation," Lambert said during the deposition.
Lambert couldn't identify the other people, acknowledging that his testimony could stigmatize those individuals.