What letter was he talking about in the first episode of The Mentalist?

Just watched the mentalist season 1 episode 1 probably one day ago and couldn't understand why the doctor who killed in the first episode posing as red john was talking about the letter. Did he also wrote a letter for his murder which I didn't notice. Or was he talking about the letter the real red john wrote after murdering patrick jane's family? Why would he refer to a letter he didn't wrote? If anyone can explain by understanding what i am saying please don't give me spoiler I just watched the first episode.