“Please, you must go away,” he said in a low, trembling voice, as the students stopped chatting and listened in. “I have nothing to say, except that if they want to come back, they can,” he said, referring to his daughters.

“If they don’t,” he paused, and flicked his hand as if to brush them away. He turned his back without finishing his sentence.

The trauma is particularly acute among Somalis. Somalis are struggling with the lure of Islamic extremism on multiple fronts: More than 100 Britons are thought to have joined the Shabab, an Islamist group in Somalia, according to the intelligence services.

A person who knows the Halane family said that another one of the children, a son, had gone to Somalia to fight with the Shabab, but then moved to Syria and joined the Islamic State last year.

Ms. Jaffer said the stigma faced by families was acute. Families know they are gossiped about and shunned. Some siblings refuse to go to school because they are afraid of being bullied.

Part of her organization’s work is to help the families rejoin their communities by persuading others they will not be punished if they show support. But so far, the convincing has been difficult.

Activists said governments were making the problem worse with their plans for tighter antiterror laws, more stop-and-searches, and the 12-year prison sentences handed down in Britain recently to two returning jihadists after their families cooperated with the police. The policies are discouraging others from coming forward, community activists say. The police have arrested 271 people on terrorism-related charges so far in Britain this year.