Ask to see a release of information from the patient stating that they give you permission to talk to said men in black
Seriously, tell them that you are unable to discuss the patient's care with anyone without a written release, and that if it pertains to any potential legal action, you will only answer questions with your attorney present. Then refer them to your nurse manager if they keep pressing the issue.
If they start making threats about how you're being uncooperative or breaking the law...well, only a judge can rule if you've broken the law, so that's an empty threat.
And don't let them insinuate that if you need a lawyer to talk to them that you must be guilty of wrongdoing...better they think that and you're protected legally by your lawyer's presence, than you trying to show that you're innocent by cooperating without legal protection for you.