That would be Muslims specifically, and middle-eastern Muslims generally.
The problem is, not all of them look like that. Chechnyans look like Europeans. One of the recent London bombers was Jamaican. Jose Padilla is Hispanic, and the people arrested in that cell in Washington state were black Americans.
But most of them are middle-eastern Muslims, mostly males, under age 40. So that's a good place to start.
Profiling or applying filters to the vast "noise" in order to increase the opportunity to find what you are looking for, is applied in all forms of research. Why not for finding terrorists? I agree that there are a small percentage of terrorists who don't fit the skin color filter, but the majority do. However, it is just one tool or filter of many. If we completing excluded everyone who wasn't a young Arab male, then we would be foolish. However, not spending most of our effort on young Arab males is also foolish.