With modal verbs, don't think that tense equals time- we can use past tenses to talk about the future and present tenses to talk about the past. The distinction between may have and might have is being eroded, but when talking about a past event, we can use both:
There was a plane crash and many may have died. (= it is possible that they have died, but we don't know yet)
I might have been killed in the accident. (= there was an accident with a risk of my dying, but I lived)
However, this distinction is becoming less marked.