The girls, all 15 to 16 years old and said to be "straight-A students" from the same east London school, disappeared from home Tuesday without leaving any messages. Authorities said they boarded a plane to Istanbul.
The relatives of Shamima Begum and Amira Abase, both 15, and Kadiza Sultana, 16, broke down in tears as they spoke of their shock and fear in televised interviews on British media.
"We miss you. We cannot stop crying," said Abase Hussen, Amira's father, clutching a teddy bear Amira gave to her mother on Mother's Day. "Please think twice. Don't go to Syria."
The case has captured wide attention in Britain, where authorities have warned that the threat is growing from Britons travelling to Syria to fight with Islamic State militants. Officials say at least 500 people have left for Syria, and police say there is a growing trend of young girls and women interested in joining the group.
Authorities have been criticized after it emerged that, before the girls disappeared, Begum had had online contact with a fourth girl who left for Syria in 2013 to become a "jihadi bride" — to marry a militant fighter.
All the families said there were no signs that the girls were interested in extremism or of what they were planning.
Scotland Yard said its officers were working closely with Turkish authorities in their investigation.
Begum's elder sister, Renu, said Shamima was a "sensible girl."
"She's a clever girl, but she's only young. And young minds can easily be swayed," she said. "You're our baby. We just want you home."
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