Well, FBI or rangers could potentially be the ones searching for him, depending on what he's wanted for. In my research for my last one, I found that the FBI was the one generally doing the trying to bring the guy in for certain types of cases. Even if he's committed crimes out of the country, if they're things that he's actively being charged with and there are extradition treaties, someone would likely be searching for him.
Unfortunately all this is research I did three years ago, so it's kind of rusty in my brain. :/
ETA: Oh, do you mean like who would he be working for? This is one of those cases where I think we're allowed to take liberties. The CIA has had assassins before, of course, but I think if you come up with a logical, realistic sounding reason you could do just about anything. He could probably even be working for a politician or someone if you wanted him to be.
Ooh, something that could be kind of interesting is to read up about
this group and create something similar for your story. Because it's a kind of behind the scenes sort of thing, it could be easier than having an "official" government group calling in assassinations and stuff. They just need connections to a group with lots of power and ambition that's willing to go to extremes, but also offer the higher ups calling for it deniability.