This achievement is one of the more difficult, and missable, of this DLC.
In order to do this you need to find two pocket secretaries prior to letting Mejia out of solitary. After the riot starts and you head toward the main administration building, there is a central internal courtyard between the two main towers. At the bottom of the courtyard, in the center, is a white security building (if coming from the side with the morgue and solitary, get off the elevator at Observation...you'll see it over the balcony, one floor below you). Inside this building, you'll find a pocket secretary inside a desk drawer about changing Mejia's work schedule.
To get the second pocket secretary, take the elevator that goes to the morgue and to solitary confinement all the way to the top floor (the floor is automated defense or something like that). Go into the security room more or less across from the elevator doors and find an unlocked safe with a bunch of items, including a pocket secretary which discusses how Mejia had access to the drone security.
With these two items in hand, you can then let Mejia out of solitary (you can get to it directly across the bridge from where the second pocket secretary was found; dodge the guards and drones and sneak in through a vent) and confront him about the murder to unlock the achievement. Without both of these items you can accuse him, but he won't admit to it.