Often, a mention of the family’s influential tribe, the Dulaimi, is enough to protect them. This time, it was no help.

Capt. Jay Ostrich, a spokesman for the United States military in Iraq, expressed condolences to the victims’ families and said American forces were prepared to aid in the investigation if the Iraqi government asked.

“By no means were American service members involved,” he said in a statement.

Details of the attack remained sketchy on Saturday afternoon, as the Iraqi police cordoned off the neighborhood and ordered a curfew there. The known survivors are all children.

According to accounts by relatives of the victims, neighbors, Iraqi security officials and others, as many as a dozen men wearing what resembled American and Iraqi military uniforms arrived in the neighborhood in a minibus and Iraqi military and police vehicles Friday night.

Witnesses, speaking on the condition of anonymity, said some of the men wore black masks and carried weapons that resembled M-16 rifles, which are used by the American military.

They knocked on the doors of several houses and gathered people at the home of Saif Shaker, an Awakening Council member.

Image Iraqi soldiers stopped vehicles at a checkpoint in Baghdad on Saturday as part of security measures after killings in the south district of Hur Rijab. Credit... Sabah Arar/Agence France-Presse — Getty Images

The men said they were part of a joint unit of American and Iraqi soldiers investigating a crime.

Some of the men spoke English translated to Arabic by an interpreter who told the family that the adults — men and women — should go upstairs, while the children should be left downstairs. They were told not to worry.