In Netflix TV show "Mindhunter", Holden and Tench go around and interview serial killers:
- The very first killer they brain-pick is Ed, who is very chatty and befriend with the two FBI agents somewhat.
- Another serial killer they interview is aggressive and in denial of all his convictions, by the end of their first interview, he says something similar to"Ed told me you guys are idiots and you must have suck every single line from him"; which heavily suggests Ed and this shoe-killer have been talking.
Maybe I missed something but how they manage to talk to each other? As convicted serial killers, their communication must be closely watched, including phone calls and letters, letting alone this shoe-killer would not have learnt Ed had been interviewed through an official channel.