LONDON >> Turkish police searched Monday for three missing British schoolgirls believed to have gone to Syria to join the Islamic State extremist group and their families issued emotional appeals urging the girls to return home.

The girls, said to be "straight-A students" from the same east London school, disappeared last 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 loss 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 say at least 500 people have left for Syria to join extremists and fear they pose a terrorism threat when they return.

Authorities have been criticized after it emerged that, before the girls disappeared, Begum had online contact with a fourth girl, Aqsa Mahmood, who left for Syria in 2013 to become a "jihadi bride."

Aamer Anwar, the lawyer for Mahmood’s family, argued that the police failed to engage with communities.

"I cannot see why this isn’t considered a child protection issue," he said. "These young girls have been groomed online. They have been trafficked."

In Turkey, President Recep Tayyip Erdogan’s spokesman said Monday that Turkish and British authorities were working diligently to locate the girls.

"They are working closely to find them if they have come to Turkey … and to find out what motivated them," spokesman Ibrahim Kalin said.

He called for closer cooperation to prevent foreign fighters from using Turkish territory to join the Islamic State group.

"Turkey cannot struggle against foreign fighters on its own," Kalin said. "Why aren’t they being stopped at the border? If there was information on the possibility that they may be joining terrorist activities, you should be conducting your efforts to prevent it at your border gates."

The families said there were no signs that the girls were interested in extremism or had planned to go abroad.

The police said the girls were interviewed in 2014 in connection with the disappearance of another friend, but said there was nothing to suggest they were at risk.

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Suzan Fraser in Ankara contributed.