They can get past most checkpoints in Iraq because customs of modesty prevent male guards from frisking them. Their flowing black garments hide something deadly underneath – explosives that can, and do, kill scores of people around them when detonated.

"They" are female suicide bombers. Except, they don’t just kill themselves. Four such bombers in two Iraqi cities brought explosives into heavily secured areas this way on Monday, killing at least 51 people (other reports say at least 57 to 61 died) and injuring 250 more – many of the victims women.

One of the women detonated herself in the midst of a massive Kurdish demonstration in the northern Iraqi city of Kirkuk, while the other three attacked Shiites in Baghdad within minutes of each other – at a tent to feed religious pilgrims, at a checkpoint where women waited to be searched by other female guards, and in a crowd of marchers taking part in one of the most important days in Shiite Islam.

"The victims were mostly women," one police commander at the scene told The Washington Post. "I saw a woman who lost her hands and legs. She had three daughters. They were crying beside her. Their mother died later."

The Kirkuk attack triggered fighting among ethnic rivals Kurds, Arabs and Turkmens, fighting each other for land and resources in the oil-rich city, causing 12 more deaths.

U.S. military officials and some local Iraqi police blame Al Qaeda in Iraq, a Sunni insurgent group. Insurgents are increasingly using female bombers because their black robes easily hide explosives and they are less likely to be searched.

U.S. military figures show at least 27 female suicide bombings this year, compared with eight in 2007. It’s because of these figures that the U.S. has stepped up efforts to recruit and train women for Iraq’s police force and enlist them to join Sunnis fighting Al Qaeda. About 200 women were deployed by Iraqi security forces this week to help search female pilgrims heading toward the shrine of an eight-century Imam in Kazimiyah, but the attacks instead took place about six miles away. There were too few women guards to search every female in the procession.

Police officers at the scene told The New York Times that authorities had heard that six women would blow themselves up in the area.

"We can’t search women," officer Atheer Allawi said. "They are wearing abayas, and God knows what they can hide under them."