The prison building is the closest building to you as you enter the Radio station area at the top of Black Mountain.
Raul Tejada, Black Mountain's ghoul mechanic, is imprisoned in this building. The door to his room is locked by the terminal right next to it. You must either have a Science skill of 100 to hack the terminal, or a Lockpick skill of 100 to lockpick the door. Or, you can unlock the door by accessing the second terminal near the building's entrance, and reading entry 6. This gives you the password, which is 123456789, to unlock the door without hacking the terminal.
Once freed, Raul Tejada can be recruited as a companion.