Iraq, where thousands more foreign ISIS followers are being held, has sent a little less than half of the 1,060 foreign children it was holding back to their home countries. Of those repatriated, 188 children of Turkish women who joined the Islamic State, as young as 1 and as old as 16, were returned to Turkey last week. Iraq has sentenced some of their mothers to death for their membership in the group.

The Americans repatriated from Syria this week wanted to go back and did so “without any pressure or coercion,” Abdulkarim Omar, a senior official with the Kurdish administration in northeastern Syria, said in a statement. He did not identify the Americans or provide any other details.

It was not clear whether they would face prosecution when they came back to the United States, or what would happen to them.

Ms. Polman said the Americans had originally been a family of seven: a mother, father and five daughters, who were still children when they went to Syria. The parents and eldest daughter were killed in Syria, she said, leaving four younger daughters, two of whom married ISIS fighters and had children.

The four surviving sisters and four children are now being repatriated.

The Islamic State, which once controlled territory the size of Britain across Iraq and Syria, lost its last scrap of territory in late March, after a four-year military campaign to defeat it.

As the Syrian camps overflowed last winter and the Kurdish authorities and humanitarian groups alike struggled to shelter, feed and care for thousands of ISIS followers and their families, foreign governments were forced to weigh the futures of their citizens with new urgency.