Well, on top of McVeigh being on those 'SSRI's' he was a patient of a doctor that just happened to be the TOP DOCTOR involved in the 'mk-ultra' experiments... I honestly don't know very much about this, but my understanding is that the film 'manchurian candidate' is a litteral possibility under the types of processes they put the victim through.
Now, it may be unlikely, granted... but he was a psyciatrist in the millitary (so there is the potential of being exposed to the types that would be involved in that type of project / operation), and having word of him discussing issues / alluding to his plan / antiwar sentiment, etc... are all, from my understanding, 'symptoms' that you would see in that type of case.
I, like anyone not directly involved, lacks the information to say this concretely, but if the man wakes up and claims to have no idea why he's being held, would be the next sign...
Look, I'm not trying to get into the whole 'conspiracy' aspect, but since these programs are known to exist and the circumstance, the question must be raised.