"The rapid, and unexpected, gains suggested a shift in tactics by the Sunni extremists of the Islamic State, also known as ISIS or ISIL, or perhaps a sign of their weakness, as they abandoned their dug-in positions and regrouped in western neighborhoods. That allowed thousands of civilians, which aid groups had said were being held as human shields, to flee across two bridges over the Euphrates River beginning on Thursday.... Tens of thousands of civilians have now fled Falluja since an offensive began late last month, and most have reached bare-bones camps that are running low on supplies.... The latest exodus of civilians has not only added to the grave humanitarian crisis that has been unfolding for weeks in Anbar Province, but it has also presented a serious security challenge to the Iraqi government’s ability to carry out security screenings of civilians so Islamic State militants are not able to escape by blending in with civilians.... But with so many people fleeing in recent days, the ability of the security forces to carry out adequate screening has been strained, raising concerns that militants are also escaping amid the flow of the civilian population...."
I added boldface to that text, which I selected from a NYT article titled "Iraqi Forces Enter Falluja, Encountering Little Fight From ISIS."