May have should be used only when you are not sure whether or not something happened. If you want to say that at some time in the past it was possible for something to happen but in fact it did not, use might have.
• An accident in which two people may have drowned happened in a village yesterday.
This sentence implies that you do not know whether the people are alive or dead. If you say two people might have drowned you are implying that they survived, although the accident could in other circumstances have led to their deaths.