This wasnt really sent by JFK Jr. This was a letter sent to Biden in the mid 90s and the FBI investigated. The actual sender was a 62 year old man from Worceseter, Mass upset about a crime bill Biden coauthored. He signed the letter as JFK Jr, not sure if it was a joke or if he was just mentally unwell. FBI investigated and were able to pull fingerprints as well as indented writing (which means basically invisible writing left from indents. Say you had a stack of paper. You hand write a letter to your mom on the top piece. Then a few days later you take the next piece, put it in your typewriter and type a threatening letter. The FBI checks to see if there are any indents left from other writing). In this case, the author had filed out something for an insurance company that left indents and led back to him.
The case documents are online from a Freedom of Information Act request. Files for this particular one start about 150 pages in. https://vault.fbi.gov/John%20F.%20Kennedy%20Jr./John%20F.%20Kennedy%20Jr.%20Part%201%20of%201
It comes from a Freedom of Information Act dump. The letter is real, but the sender isn't really JFK Jr. The pages relevant to this begin around 150. Actual author was a 62 year old dude from Massachusets.
https://vault.fbi.gov/John%20F.%20Kennedy%20Jr./John%20F.%20Kennedy%20Jr.%20Part%201%20of%201