Lawrence Phillips complained that prison staffers were screwing with him in the month before he died ... messing with his mail and breaking his TV ... according to a letter penned by the ex-NFL star.
TMZ Sports obtained the letter Phillips sent to his old high school football coach on Dec. 6th -- in which he implied staffers at Kern Valley State Prison were NOT delivering his mail.
"The mail situation is terrible," Phillips wrote.
"Everyone is pointing the finger at everyone else. I surely am not receiving all my mail as well as mail I am sending is not being received. There were two letters you mentioned that I never received. This new attorney has said he is addressing the issue."
There's more ... in the letter, Phillips implies prison staffers also broke the TV in his cell ... a 1-man cell that was segregated from the general population.
"I am allowed a television or a radio. They broke my television so I am listening to the radio. I get a lot more done without the television anyway."
As we previously reported, the prison says Lawrence committed suicide early Wednesday morning -- but his close friends and family say they don't believe it because he was in good spirits despite the fact he was facing the death penalty in the killing of his former cell mate.
It seems to line up with the letter in which he engages in some fun banter about college football -- and pondered about Jon Gruden possibly coaching at USC.
When it came to his case, Phillips noted he was confident he would be found not guilty of the crime.
We called the prison for comment -- but were told "everything associated with this is being investigated and we can't comment on anything at this time."