Just because they didn't succeed, doesn't mean they're not guilty of trying. It has been established in bio-weapons research that they have a traditional problem in creating an effective pathogen - they can make it deadly, or they can make it highly communicable, but they can't do both. Doesn't prevent them from continuing to try. Besides, creating an artificial pandemic out of fear accomplishes the same goal. For example: they could directly infect a few people with a very deadly strain, knowing that it won't continue to spread - but make a big clamor about the few they were able to kill, and occasionally repeat this with a handful of select people here and there to continue the ruse that it's spreading.