There are reasons to believe that Doctor B, who previously worked with A, may know A's location. Unfortunately he's adamant that he can't answer questions about A. Although he doesn't explicitly say so, the investigators suspect that he's A's doctor, or has been in the past. B has no direct evidence that A is a criminal, everything he knows comes from the agents etc., but knows enough about A to suspect it might be a possibility.
Since this isn't 24 the agents have to pay some attention to the legalities of things, so drugging or beating the information out of B is out of the question. Arresting him or causing any sort of fuss will probably warn A that they are getting close. They know how much of the drug A has, and that a lot more people may be hurt if more is sent out.
I need a quick argument from the agents (one of whom is yet another doctor) that will help them overcome B's reluctance to talk. I think this is going to have to come from medical ethics. This needn't be terribly long or valid legally - I just want something that sounds convincing enough to get past the ethical dilemma in B's mind.
One obvious question; would someone's address even be covered by confidentiality?
All of the case examples I can find relate to sexually transmitted diseases, gunshot wounds, etc. where the doctor sees clear evidence of criminal behaviour, which isn't the case here, or to the US HIPPA laws.
* Medical Ethics confidentiality
* Medical confidentiality
* Doctor reports patient criminal behaviour
Update - it appears that revealing the address would be OK, that's not covered by confidentiality. Many thanks to those who commented.
another update - it's a greyer area than I thought, but I think I can write my way around it.