Was there an assassin on Grant's train?

[RJNorton]

My main curiosity here is the identity of the messenger. Just who was it? John Fazio is convinced it was David Herold (sent by Booth). Some Lincoln authors seem to think it was a legitimate but poorly dressed courier sent by Mary Lincoln. Is it possible that it could be another person sent by Booth (or another Confederate operative)? Not Herold, though. Could it have been the "mystery man" who followed Grant onto the train with intent to assassinate? And later sent the letter to Grant?