My google-fu is failing me. I'm getting a lot of info about the U.K. (apparently there was a bill about witness anonymity in the news there), but not really any about the U.S.
Setting: Modern day, U.S. suburbs
Google/Wiki: Witness responsibility, witness procedures, witness privacy, witness anonymity
Situation: Man A is getting into his car when Man B walks up and asks him for a lift, hinting that he's in trouble. Man A refuses, and several days later learns Man B has been murdered.
1) Is Man A required to contact the authorities and mention the encounter? He'll have an alibi for the time of death, but is uncertain about stepping forward. If he does, I'm assuming he'll be put into contact with the investigators and asked to come in and give a statement.
2) How likely is it that Man B's family/friends would find out Man A's identity? Is that info part of the public record when it's an open investigation, or does it remain undisclosed until/unless someone's caught and there's a trial? Does Man A need to worry about being confronted about why he didn't help?
Updated: Thanks to everyone who replied!