should the verb that comes after interrogitive pronoun be singular or plural if the noun is plural?
some people were killed
who were killed?
who was killed
they are happy
who are "they"?
who is "they"?
what is happy?
Oh dear. This may seem like conflicting information.
Who was killed? (Without a specific plural noun, even if you know more than one were was killed, use is. Otherwise, you can say "Who were the people [obviously plural] who were killed?"
Who are they? ("They" is obviously plural, so use the plural verb.)