Three schoolgirls are feared to have run away from east London with plans to travel to Syria and join Islamic State.
Police said the close friends were last seen on Tuesday morning as they left their homes telling their families they would be out for the day.
Instead they met and travelled to Gatwick airport before boarding a Turkish Airlines flight, which landed at Istanbul that evening.
The three - Shamima Begum, 15, Kadiza Sultana, 16, and a third female aged 15 who is not being named at the request of her family - are pupils at Bethnal Green Academy.
Metropolitan Police Commander Richard Walton said he was "extremely concerned" for their safety.
He revealed the runaways are good friends with another 15-year-old girl who fled to Syria in December.
Kadiza Sultana, 16, Shamima Begum, 15, and an unnamed 15-year-old
"We are concerned about the numbers of girls and young women who have or are intending to travel to the part of Syria that is controlled by the terrorist group calling themselves Islamic State," Mr Walton said.
"It is an extremely dangerous place and we have seen reports of what life is like for them and how restricted their lives become.
"It is not uncommon for girls or women to be prevented from being allowed out of their houses or if allowed out, only when accompanied by a guardian.
"The choice of returning home from Syria is often taken away from those under the control of Islamic State, leaving their families in the UK devastated and with very few options to secure their safe return."
Mr Walton added the teenagers' families were "devastated" but there was a "good chance" the girls were still in Turkey.
All three have mobile phones, and police are using Turkish media and social media in the hopes of reaching them.
Salman Farsi, spokesman for their local East London Mosque, said: "They have been misled. I do not know what was promised to them. It is just sad. We have not had anything like this before in our community.
"I think the girls need to know they have done nothing wrong. They have been manipulated."
The girls are students at Bethnal Green Academy, east London
A family friend of one of the missing girls told Sky News: "It's really sad what has happened.
"Maybe they need to educate people more, tell them about the risks of going to Syria, that it's not safe out there."
The number of Westerners who have travelled to Iraq and Syria to join IS is thought to be about 3,000, including as many as 550 women, according to the London-based Institute for Strategic Dialogue.
Former Metropolitan Police border control officer Chris Hobbs told Sky News that checks for people departing from UK airports made it a "walk in the park for jihadis and girls like this" to leave.
"At the moment you go through security, you get on the plane, you might be checked by a private security guard," he said.
"Unless you're very unlucky you won't pass under the eyes of anyone from UK law enforcement.
"If you're on a watch list then you will ping the system. If you're not on the radar then the odds are you will get on the plane without too many problems."
Police have released descriptions of the girls:
:: Shamima Begum, 15
Shamima is around 5ft 7in tall and was wearing black, thick-rimmed glasses, a black hijab, a light brown and black leopard-print scarf, a dark red jumper, black trousers and jacket.
She was carrying a dark blue cylindrical holdall with white straps. She is a British national of Bangladeshi heritage and speaks English with a London accent. She also speaks Bengali.
:: Kadiza Sultana, 16
Kadiza is described as 5ft 6in tall and slim, and was wearing black-rimmed glasses, a long black jacket with a hood, grey striped scarf, grey jumper, dark red trousers and was carrying a black holdall.
She is a British national of Bangladeshi heritage and speaks English with a London accent. She also speaks Bengali.
:: Third Missing Girl, 15
The third girl, who is not being named, is German but living in London. She is described as 5ft 6in tall and slim. She was wearing black, thick-rimmed glasses, a black head scarf, a long dark green jacket with a fur-lined hood, a light yellow long-sleeved top, black trousers and white trainers.
She was carrying a black Nike holdall. She speaks English and Amharic.
