Start from Primm in the direction of Nipton in the South-East, when you arrive in the city, a man apparently very happy will accost you (Picture 1). The winner of the "lottery" leaves and you enter the mass grave which was called Nipton. Approach the Legion men posted in front of the town hall (Picture 2), the leader of the torturers will then give you a message to pass on to the NCR men at the Mojave outpost. Refer to side quest “A heart cold as ice” for more details. After the departure of the men of the legion, continue your journey north to reach the small town of Novac (Image 3).
Enter the City Motel and then go visit Manny Vargas in the first room on the ground floor (Picture 4). The young man seems to know a lot about the men who assaulted you but won't say anything until you do him a favor. If you are computer savvy enough, you can hack Manny's computer to get new information regarding the main quest (Picture 5). If not, refer to the quest "Fly with me" to get Manny's info.
So you have learned that the Khans stationed in Boulder City have information that may be of interest to you. Start from Novac and continue your journey north to reach the town of Boulder City (Picture 6-7). When you arrive, go talk to Lieutenant Monroe to get the side quest "The Boulder City Show" (Picture 8). By completing this quest you will get the identity of your attackers and end the quest "They went that way".