He could have been part of a second-wave bio or chem attacks, something the govt. has been very reluctant to elaborate about, not unlike their touchiness about labeling any post-9/11 airline incidents with terrorism, a bow to the airline lobby IMO. Naming Hatfill as a "domestic" perp also gave a false sense of security - a sole actor, wrapped up with innuendo who is no longer a danger in the future.
M's case might implode the various Fed dissembling - moving him to a secret tribunal reinforces might be motivated by avoiding this uncomfortable eventuality.