The following is my assessment of the envelopes addressed to Senator Daschle, Tom Brokaw and Editor. My only source was the FBI website. At first analysis the writing had enough variation to have been done by different individuals. With more careful analysis I have concluded that three envelopes were written by the same individual but with a progression of a loss of control, and quite possibly an individual on a suicide mission and or under the influence of some chemical, possibly anthrax.
I feel strongly that the envelopes were written by the same writer and may not have been written in the same order they were mailed. The writing may be that of an individual who is a rather simple type of follower, not intelligent or broad enough to have this calculating kind of ability on his own.
In his mind in writing the Daschle letter, he was outwardly over controlled and with simplistic thinking. A likely pawn, with tremendous inner mood fluctuations. Likely accustomed to following orders, rather than giving them.
In the Brokaw and the Editor envelopes the writer is either cracking up from the pressure of the reality, or alternatively he is drunk or very ill. In these later letters he appears to be hypnotized, or is hypnotizing himself to go through with what he has committed to do. He is also very afraid and is pressing the gas at full throttle to be courageous and overcome the fear.
In any case, the writing progressively deteriorates, looses the elasticity and neuro-motor control, as he becomes ever more rigid, disturbed and blinded to reality. He is heading to the edge, almost as if repeating a mantra or marching to a rhythm (compulsive behavior), this fear and anxiety and the repetition builds his force to carry through and to drown out the terror and rage.
It could be that he was drinking or actually becoming hypnotized with his repetition in his head. His inner pressure was building force and he became more determined and bold, while his thinking and body are both progressively deteriorating, unable to contain himself and going over the edge.
I see patches of his former self that lead me to believe that this is the same writer in a severely disturbed state of mind.
This is my initial analysis from the website samples.