Dev Question: How is Damage/Death Calculated?

So my question is: When a player dies, it is tied directly to damage being dealt to them that equals or exceeds their health after damage reduction. When this damage is dealt is due to whatever the source is, which is then given credit for the kill: such as a champion ability, or neutral jungle monsters. This system also includes an assist system in which damage dealt leading up the death, but not the cause of death, also gets acknowledged and rewarded. The system would be able to determine which damage was lethal, probably up to the millisecond, of which ability landed when. For argument sake, let's say that multiple sources of damage from multiple original sources were to land at the exact same moment, and each of them was enough to kill the player. How would the system then dole out the kill and assists and for which abilities?

To explicate:

Player A has 100 HP

Player B hits player A with a skillshot that does 200 damage at exactly 60 seconds into the game. Meanwhile Player B also hits player A with an AOE that deals 200 damage as well simultaneously.

While this is going on Player C also simultaneously hits player A with a targeted ability that does 200 or more damage.

Meanwhile Player D Ignites player A and the first tick of damage goes off simultaneously with all the other attacks.

How does the system determine whether Player B, C, or D gets the kill and to which of the abilities the kill goes?