No one involved that day expressed any doubt about Ivins’ guilt. But things are not always as clear-cut as they may seem in an FBI presentation. Two years later, sitting in her office overlooking West Baltimore, Fraser-Liggett concedes she has reservations. “There are still some holes,” she says, staring out her window in discomfort. Nearly 2,000 miles away in Flagstaff, Arizona, Keim has his own concerns. “I don’t know if Ivins sent the letters,” he says with a hint of both irritation and sadness. Even agent Edward Montooth, who ran the FBI’s hunt for the anthrax killer, says that—while he’s still convinced Ivins was the mailer—he’s unsure of many things, from Ivins’ motivation to when he brewed up the lethal spores. “We still have a difficult time nailing down the time frame,” he says. “We don’t know when he made or dried the spores.” In other words, it’s been 10 years since the deadliest biological terror attack in US history launched a manhunt that ruined one scientist’s reputation and saw a second driven to suicide, yet nagging problems remain. Problems that add up to an unsettling reality: Despite the FBI’s assurances, it’s not at all certain that the government could have ever convicted Ivins of a crime…

Montooth acknowledges that he isn’t sure how Ivins would have done all that growing and drying. “But it almost doesn’t matter,” he says. Investigators know which days in September and October the envelopes were mailed. That was the actual murderous act. The anthrax could have been slowly assembled and processed for months or years before that. Ivins’ alibis for those autumn days are virtually nonexistent.

There are still other problems with the case against Ivins. The killing spores were so volatile that they cross-contaminated piles and piles of mail. Yet spores were never found in Ivins’ house or his car, and only a handful were discovered in his lab. There’s no evidence of any trip to Princeton to mail the letters. And just because the killer spores were descendants of a USAMRIID flask, there’s no guarantee a USAMRIID scientist was actually the mailer. In fact, the FBI was never able to prove where the attack anthrax was cultured. “It would’ve been very easy to take the anthrax out, to steal some,” a former USAMRIID officer says. “Anybody could do that.”

Finally, there’s the matter of motive. The Justice Department asserts in its investigative summary that Ivins mailed the letters to gin up support for an anthrax vaccine, offering a few ambiguous emails and comments to friends and investigators as proof. If there’s any further, credible evidence to support this notion, Wired couldn’t find it in the thousands of pages of case documents released by the government or in the hours of interviews conducted with the investigators. Montooth concedes it’s a placeholder rationale at best. For someone as deeply disturbed as Ivins, he argues, simple rules of cause and effect don’t apply, especially not in matters as grave as murder. “You cannot think of this in one dimension or layer. It’s not that simple,” Montooth says. “You’re never gonna know a single cause or motive for why it was done.”