Suicide bombers, including at least three women, struck Shiite pilgrims in Baghdad and Kurdish protesters in the northern city of Kirkuk on July 28, killing at least 57 people — a brutal reminder that mass gatherings remain vulnerable despite improvements in security.

The attacks came even though the United States has stepped up efforts to recruit and train women for Iraq's police force, and enlist them to join Sunnis fighting al-Qaida. Insurgents increasingly use female bombers because their billowing, black robes easily hide explosives and they are less likely to be searched.

The three nearly simultaneous bombings in Baghdad undermined public confidence in recent security gains that have tamped down sectarian bloodshed. The attack in Kirkuk showed that ethnic rivalries can turn into mass slaughter in a city that is home to Kurds, Turkomen, Arabs and other minorities.