This is going to be a bit of a longshot, I suspect. I started reading a book in the 2010 timeframe (probably around 2015ish) that was, I think, at least the second book in the series. It started with an Islamic man (an Imam, maybe?) being assaulted somewhere in the Middle East, and being saved by a man in ragged clothes who turns out to be an American ex-soldier who went AWOL after having a change of heart about fighting. Said soldier becomes a loyal disciple to the Imam, who apparently has terrorist leaning. The next scene I remember was one at a terrorist training camp with the main character being a well-educated Arabic young man who'd previously studied in the United States, and is now studying things like explosives, I think under the tutelage of the ex-soldier, who is quite the fanatic now. A few chapters later (or maybe the camp was a flashback), the young man is working concessions at a major sports stadium in the United States, where he has befriended a young woman (of European descent, I want to say Irish), and he's expressing internal regret that he won't be able to warn her about the upcoming attack, which I think involves smuggling explosives being smuggled into the stadium to be detonated during the game.
I want to say that none of these were the main characters of the story, but rather the antagonists to the protagonist, who might have been a detective rather than, say, a military operative. Unfortunately, my brain gets fuzzy there.
I don't remember if this was a paperback, a hardback, or an audiobook. I checked it out from a library in Bridgeville, PA, I think, but I check out a lot of books, so my attempts at scanning my history haven't borne much fruit. I think I was getting into David Baldacci at the time, having read Memory Man, but the book summaries on Baldacci's site are not sparking memories. The other book I remember reading at the time was Executive Privilege by Philip Margolin.