A fairly easy achievement, but can only be gotten at the end of the game. There are two ways you can go about getting this achievement, so it's hard to miss.
The soonest you can possibly get this achievement is right after getting out of the Medical Bay in the final mission. Once you go through the elevator, you'll come to a hallway that forks. After taking care of the first few enemies, go into the room into the right, and go to the big screens. When you interact with it, you will be given the choice between downloading data, and contacting Alan Parker. Choose the former, and the achievement should unlock shortly after.
NOTE: In order to even get the option to download the data, you MUST spare Omen Deng. Otherwise, it'll cut straight to you talking to Parker.
The other way to go about this is when you get out of the Medical Bay, thoroughly look through every room you can until you come to the interrogation room. If you have romanced her, or have +6 like with her, you will find Scarlet here. Talk with her, and after a while, she will tell you the identity of Sung's assassin.