The families of three British schoolgirls believed to have joined the Islamic State group in Syria say police let them down by failing to contact parents directly during an investigation into the disappearance of a classmate.

London's Metropolitan police gave letters to the Shamima Begum, 15, Kadiza Sultana, 16, and Amira Abase, 15, to take home, asking their parents for permission to take statements about another friend at their school who vanished in December. The families found the letters hidden in the girls' textbooks.

Halima Khanom, Sultana's sister, told the BBC that if the family had known what was going on, they would have "taken precautions with my sister."

The police said Saturday that there was nothing to suggest at the time that the girls themselves were at risk.