Get Killers Weapon

How may I go about finding a persons weapon? For example: a player kills another player. I want to know what weapon the killer is using or what item is in that persons hand. Thanks for your help

You can get whatever's currently in a player's hand with $player->getInventory()->getItemInHand();

Use EntityDamageByEntityEvent

Note that if a projectile hit the player, you should check the getChild entity in EntityDamageByChildEntityEvent, because by the time the projectile hits the player maybe the player is no longer holding the bow/snowball/egg.