Complex sentence: When it reaches the ground floor, Trooper Barrigan (James Badge Dale) shoots Costigan in the head, then shoots Brown, and afterward reveals to Sullivan that Costello had more than one mole in the police.
Split into simpler sentences: When it reaches the ground floor, Costigan is suddenly killed by Trooper Barrigan (James Badge Dale) moments before the later shoots Brown upon his arrival. He then afterward reveals to Sullivan that Costello had more than one mole in the police.