Following the last episode, the murderer (Aidan McArdle) presumably committed suicide by hanging in the cell, much to the relief of victims' relatives (they even set up a support group). Anyway there is no chance a cord could be taken to prison without anyone noticed. Someone with proper cover had to take it from outside. CCTV set Casey up, because he was the last person who get in the cell and he had motive, due to the anger felt against the killer. But he's not the only one with access to the joint and somebody could have tampered CCTV footage.
Most of the episode focused on main character personal lives: Thorne has just lost his mother (from brain cancer) but he's not willing to go to the funeral because he hate her; Casey is still freak out for the train accident and he struggles to deal with his son's custody, leading him to drink heavily; Brooks hasn't understood yet family is more important than job in his daily life.