They weren't Americans - and yet their lifestyles were similar... in some ways. But apparently that was enough.
Al Qaeda - Osama bin Laden - chose mostly Saudis to carry out their attacks on the WTC and the Pentagon. And yet, because they worked with Americans or just because they lived a somewhat more secular life than OBL would approve of, al Qaeda "expressed its displeasure" with their Saudi brethren.
But these were not soldiers or policemen. They were women, men, children; at home in a quiet, residential, supposedly secure area. 11 dead. Perhaps as many as 110 wounded.