Special Relativity.

There were 5 students in a class. One student killed another student. Many many years later ( respect to Earth frame) those 5 students are all gone (death) and you decide to find who was the killer in the class. You don't know who the killer and who was killed too.

Your task is to find out who the killer was and which person was killed. The question is: at least how many observers -who have different velocities respect to earth frame- do you need to ask in order to figure out which student was the killer and who was killed?

(I know this is a little bit strange question but I don't image now a different example to illustrate this case.) Hint, this question is about special relativity, more specifically it is about causality.