I'm only on season 3 episode 17.  And after reading some theories I do believe that there is some possibility to there being multiple people versus just one.  But what makes me think it is Bertram is at the end of episode 16, season 3 Bertram recites a poem to Lisbon.  Bertram recites a poem, smiles and then says "just a little poem I like" and walks away.  LaRoche says, " William Blake, I believe"  The verse that is recited to Jane, "Tiger, tiger.." is also William Blake.  Coincidence?  I think not ;)