Under HIPPA, yes, you have a Duty To Warn, but only if the patient names a specific person. And then you can’t tell anyone who the person making the threat is.
Cute, huh?
More crap from the people who make saving endangered snails a federal priority. Thanks to "privacy" laws, we learn a great deal more about victims than we do about criminals, even those who commit the most horrific crimes. The result is that we are unable to take reasonable measures to protect decent citizens since we are left with no idea of what to look out for in the future.