While he was in solitary:
1. why did the guard give him so much grief about mentioning the FBI? ("I don't ever want to hear you say FBI again." or something to that effect.)
2. instead of asking for a pencil, wouldn't it have been simpler to say something like "I want to confess to murder and give the locations of the bodies." No way would they ignored that. He'd have plenty of opportunity later to put it in context.
Well i am not American but what i have concluded from watching so many movies over a long time that their police department and FBI don't bond that well. They are working same but FBI working higher in ranks take uneven advantages and often sideline police's work. So, police hates them and they are justified in doing so.
If he had confessed to a murder, he would have been tried for that and he would end up being convicted. He was already in pain and then he lost his phone rights and so he couldn't contact them.
Read this article and see the last lines under section "incarceration" and you will find the answers.
https://en.wikipedia.org/wiki/Larry_Hall_(criminal)