After opening the safe and obtaining the shotgun, descend down another flight of stairs and into a dark area. Locate a body, it has a flashlight and an access card on it:

Access Card
Allows access beyond electronic locks

 Go back up two flights of stairs, then use the access card on the door to exit the guard tower. You will now be in the Cells area in section A3. Locate the door with the sign that says "Cloak Room". Open the door and enter a room with some lockers. One locker has shotgun cells in it, and another one has a walkie talkie.

Keep exploring the cells area. Enter Wing B and go down the stairs into Block A2. There is a brawler patrolling the block, so be ready to fight it. Go down the corridor and enter the first empty cell. Pull back the shelves, then examine the poster to reveal a mystery item - prisoner's letter behind it:

Babe,
Bad fucking news. Someone round here's got it in their stupid head that I am some kind of a rat snitch. Been taking heat all damn week from other inmates and the guards are coming down on me for every stinkin little thing. It ain't looking good, babe - I gotta get my sorry ass out of this place somehow. I need you to call my lawyer, let him know what's going on fast. Please. I'll see what I can do on this end - I got some ideas. Crazy ones, but better than nothing.
Love forever,
Rick.

Silent Hill: Downpour Prison Juggernaut

Keep going down the hallway. In the next cell, a Stabber is hiding so watch out. Turn right around the corner. The corridor ahead is blocked, but enter a cell to your right and find a Colt handgun on the floor. Go back and around the other way into Wing B. There is a wrench in a cell to your left. Down the hallway, you see a basketball being thrown out of one of the cells. Examine that cell. The environment will transform, and you will find yourself walking in a flooded hall. A short cutscene triggers, where you see yourself at the end of the hallway. Approach the cell at the end and find a letter on the cabinet:

Murphy,
Forgiveness, You have the balls to ask me for FORGIVENESS? How can I possibly forgive you when you've destroyed everything I had left in this world. Every time I look at you, all I can see is his face. I only ever asked one thing of you, and that was to be a good father to our son. You failed me, Murphy. You failed me and you failed Charlie, and now there is no way we'll ever have him back.
Don't call me. Don't write. I never want to see you again
-Carol

Exit that cell and turn left, as you approach the stairwell at the north end, there will be a cutscene followed by an encounter with a new type of monster: the Juggernaut.