How to NPC follow player when attacked ?

i have script npc auto follow player when players are near them. But i want it only follow player who attack them. Players use gun or skill and they stand far away and attack the NPC. How does the NPC know who attacked them?

local larm = script.Parent:FindFirstChild("HumanoidRootPart")
local rarm = script.Parent:FindFirstChild("HumanoidRootPart")

function findNearestTorso(pos)
	local list = game.Workspace:children()
	local torso = nil
	local dist = 20
	local temp = nil
	local human = nil
	local temp2 = nil
	for x = 1, #list do
		temp2 = list[x]
		if (temp2.className == "Model") and (temp2 ~= script.Parent) then
			temp = temp2:findFirstChild("HumanoidRootPart")
			human = temp2:findFirstChild("Humanoid")
				if (temp ~= nil) and (human ~= nil) and (human.Health > 0) then
					if temp.Parent.Name ~= script.Parent.Name then
						if (temp.Position - pos).magnitude < dist then
							torso = temp
							dist = (temp.Position - pos).magnitude
						end
					end
				end
		end
	end
	return torso
end

while true do
	wait(0.1)
	local target = findNearestTorso(script.Parent.HumanoidRootPart.Position)
	if target ~= nil then
		script.Parent.Humanoid:MoveTo(target.Position, target)
	end

end