How to pass the name of the killer to a command?

currently I have this skill:

- command{c="roundkills 1 <trigger.name>"} ~onDeath 1

But the result is always (unknown) for the trigger name if the mob gets killed by fire or something else.

Is there any variable for getting the next, closest player in a radius around a mob that just died?

And yes it has to be a console command (not a player executing a command)