>the US Marshall's door knock and serve subpenoas?
YES, they absolutely do. They served one for me, dangerous/threatening individual so instead of doing it alone and all by myself we enlisted them. Anon was not paid enough nor armed so not going to a proposed gunfight, that's what Marshalls do.
CI (criminal investigators) are there to allegedly protect upon request and after much paperwork but they will only protect themselves. When getting a CI escort anon made sure to stand behind, not in front of them. They would protect themselves and by extension, me too. SCOTUS -gee thanks…the ruling affected ALL who were considered LEO.
Don't start. Yes, anon has remorse…