Actually, an off-the-shelf HEPA filter (available at Home Depot or just about anywhere else) will trap >99% of anthrax spores. The perp is perfectly free to work at home as long as he keeps a HEPA air filter running nearby, and tidies up after a work session with a HEPA-equipped vacuum cleaner. Throw away the used filters and the FBI could swab his apartment for anthrax without finding anything.
Question for Oxford, CT police: What kind of vacuum cleaner did Mrs. Lundgren own? A HEPA-equipped one? If so, they need to check its filter.
Rereading that, it almost sounds as though I'm accusing Mrs. Lundgren, which is obviously not the case. Rather, a HEPA vacuum would explain how she could be exposed to anthrax inside her home without any detectable spores left behind: She'd continue to be well enough for days after exposure to have vacuumed once, which would put all the spores in the filter rather than lying around to be detected.