The question is not why he was "picked" to be investigated. The question is why he was kept so long even after it was determined that he did not, in fact, have

The question is not why he was "picked" to be investigated. The question is why he was kept so long even after it was determined that he did not, in fact, have explosives, and why he was questioned at such length and aggressiveness. The opinions of the people interrogating him are precisely what determine that, as explained by the actual person interrogating him. It really couldn't be any more straightforward.