Just a hypothesis.
Maybe anthrax was released but it didn't work(as an effective terrorist "tool"). It looks like Mr. Stevens lived in a suburban area(I do not know the population figures, but it seems he did not live in Bum, you know where, Egypt).
It seems that 1 death out of 'targeted' area of 1,000 people(again I am estimating the population of the targeted area) is not an effective "tool" to terrorize people(a 0.1% effectiveness rate). That is not a reason to get complacent, but to breathe a sigh of relief and to be vigilant.
That's the most reasonable hypothesis that I've heard in this mess.