Why not profile the 100+ other WHITE people standing there?
Because whitey does not have a long and distinguished track record of blowing things up?
Look, as a white guy, I understand that people who look like me have the market cornered on serial killings, mash shootings, hockey, etc. If I were of middle eastern descent, I think I would understand that people who bear a resemblance to me are known worldwide for their proclivity for blowing things up. While both cases only represent a tiny fraction of our given populations, the people who investigate the cases have to start somewhere.
This investigation has a limited number of resources. Those resources must be used in the most efficient manner possible to find those responsible as quickly as possible. Is rounding up a hundred random white guys going to be a productive use of time and manpower? Probably not. Is tracking down every person wearing a backpack before the explosion? Maybe.
When I get that list, and have to start rounding them up for questioning; guy #1 is Bobby O'Malley from South Boston and guy #2 is Muhammad Alawi Husain of Saudi Arabia, who should I look for first? Is that racial profiling? Should I be ashamed of myself?
Bobby could be the guy we are looking for. He could be a far right, white supremacist nut-bag who hates the fact that Kenyans dominate marathons. But like any number of other choices we make in our daily lives, you have to play the odds.