Earlier quoted context omitted.
This is the kind of target that would have someone claiming responsibility. The plane wasn't from a country that's known for disproportionate responses to attacks(i.e. the US or Russia). I would figure that if any group actually did pull this off, they'd be making public announcements that they took down the plane, or were holding it for ransom.
Aum Shinrikyo aerosolised and released anthrax for a week. They were caught. They didn't make any announcement about it. Announcing it a few days afterwards would have caused significant panic and distress. (Luckily they used the "wrong" anthrax, so not much harm done). The same organisation also did odd things in Australia. They made sarin and VX gas, tested that on sheep, and the. Used the gases in several assassin…
Also, IIRC Aum never used anthrax (although they did stockpile it), only sarin and VX, which are chemical agents, not biological.