Search terms used: "mental health facility security", "criminally insane security measures", "psychiatric patient contact", "psychiatric patient letters", "can insanity defense patients send letters"
I'm currently working on a fanfiction story, set in a universe that is fairly comicbook-y. However, I want to keep the little details realistic where I can, since suspension of disbelief can only go so far.
Character A is a recently-arrested domestic terrorist who has been placed under high security in a psychiatric facility, on the grounds that he's both mentally unfit to stand trial and very physically dangerous (but intelligent and capable of being calm when he wants to). However, he has an accomplice, Character B, who the authorities don't know about and is still on the outside.
I'm trying to figure out a way for Character B to reach out to Character A while he's confined and help him plan his eventual escape, while still concealing B's identity from the police (Character A has usually been a solo operator and voluntarily cut off from society, so the police don't have reason to immediately think someone would help him). I had the idea of them possibly exchanging coded letters that use a popular science journal as a cipher (B could send A new issues periodically), but I don't know if that's something the facility staff would tolerate, or how closely they'd scrutinize any mail that came in or out.
Most of my online searching has turned up rules for ordinary mental health patients, or else just general discussion about the rights of the criminally insane. Since this isn't set in our exact United States, it doesn't need to follow exact rules or laws, but a solid outline of what kind of outside contact and correspondence is usually allowed for known-to-be-dangerous mental health patients would be helpful.
Edit: I should have clarified: the authorities do know B's identity, just not that she's helping A. She was present when he was captured (and briefly injured and hospitalized), but they let her go because they thought she was a bystander caught in the crossfire. If she continued to have open contact with him, though, especially visiting him in person, they'd probably get suspicious.