It was just a joke, you know, I didn't mean to hurt anyone with those biological weapons:

The deadly 2001 U.S. anthrax attacks may have been carried out by someone who did not intend to cause any harm, a top FBI official said on Monday.
Of the possibility that a scientist wanted to issue a wake-up call about the bioterrorism threat and it went out of control, FBI Assistant Director Michael Mason told a news conference, "That's a possible theory, but it's all conjecture."

Asked why there had been no other attacks, Mason said, "I suppose the leading thought might be the person didn't intend to cause harm, and did."

The FBI has been investigating anthrax-laced letters that were mailed to the news media and politicians in September and October 2001, which resulted in five deaths in a country already shaken by the Sept. 11 attacks.