There has been an upsurge of violence against civilians -- mostly Shiites -- in Iraq in the last month or so, with a number of car bombings in Baghdad, for example a wave of bombings that killed 34 people on April 6 and a more recent wave that killed an even larger number. And nineteen U.S. soldiers died in Iraq in April, more than in any month since last September.
Recently an Iraqi in army uniform shot and killed two U.S. soldiers and injured three others on a base south of Mosul. Apparently the attacker was an Iraqi soldier who also worked as an imam at a mosque on the base. "Mosul is seen as the last remaining urban stronghold of al-Qaeda in Iraq," to quote the BBC report.