School friends Kadiza Sultana, 17, and 15-year-olds Shamima Begum and Amira Abase left their east London homes on Tuesday and flew to Istanbul, raising concerns they would travel on to Syria to join IS jihadists.
All three were spoken to in December by police investigating the disappearance of a friend who went to Syria but Scotland Yard insists nothing indicated they would follow suit.
Amira's father Abase Hussen, 47, said nothing in his daughter's behaviour indicated anything was wrong when she told him he was leaving to attend a wedding.
"She said 'Daddy, I'm in no hurry'," Hussen Abase told journalists at police headquarters. "There was no sign to suspect her at all."
In fact, she had travelled to Gatwick Airport to take a flight to Turkey, despite never showing signs of an interest in extremism to her family.
"She doesn't dare discuss something like this with us. She knows what the answer would be," Abase Hussen, holding a Chelsea FC teddy bear that his football fan daughter had given her mother.
"The message we have for Amira is to get back home. We miss you. We cannot stop crying. Please think twice. Don't go to Syria," he said.
Some have questioned whether more could have been done to stop them leaving.
Someone using a Twitter account in Shamima Begum's name last Sunday seems to have contacted Aqsa Mahmood, a woman from Glasgow, Scotland who reportedly travelled to Syria last year to marry an IS group fighter.
Mahmood's family, who strongly condemn her actions, have questioned why authorities did not act on the message as her social media accounts are monitored.
"She is now engaging with other young people and trying to recruit them," Aamer Anwar, a lawyer for the Mahmood family, told the BBC on Sunday.
"(The family are) saying 'what exactly are the security services doing in this country?'"
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After the girls' friend disappeared last year and was thought to have travelled to Syria, Renu Begun, 27, asked her sister Shamima: "You wouldn't do anything stupid like that, would you?"
Shamima had replied that she would not as she had the support of her family, Renu recalled.
"She's a clever girl but she's only young and young minds can easily be swayed," Renu said.
"My little sister is an A-star student. To convince such young girls at that age, who are vulnerable, it's just wrong. It's a really evil thing to do," Begum added.
Counter-terrorism experts estimate that around 50 women have travelled from the UK to Syria to join the IS group, while tales of "jihadi brides" have become a staple of Britain's tabloid press.
Some 550 women from across Europe have gone to Iraq and Syria, where they often marry fighters and help to recruit others, according to a study from the Institute of Strategic Dialogue think-tank last month.
In an attempt to stop the flow of IS supporters to Syria, the passports of Britons suspected of travelling to Syria or Iraq can be seized before travel under a new law that came into effect this month.
However, in this case the girls involved were not suspected in advance.
Prime Minister David Cameron, who plans to introduce additional measures including enhanced screening at airports, said that schools and universities had to help identify those at risk.
"The fight against Islamist extremist terror is not just one that we can wage by the police and border control," Cameron said.
Turkish Airlines, with whom the girls flew to Istanbul, said its responsibility was checking passengers' visas and pre-flight security issues were the "responsibility of official airport authorities."
Britain's first female Muslim cabinet minister, Sayeeda Warsi, who resigned last year over the government's Gaza policy, warned of the role of the Internet in radicalising young Muslims.
There is no "single journey to somebody becoming a terrorist," she told Sky News.
That was echoed by Ross Frenett of the Institute of Strategic Dialogue, who stressed more effort should be made to counter online propaganda.
"An awful lot of extremism... is people looking for a sense of belonging," Frenett said.
"That's not unique to people who are maladjusted. An awful lot of extremism of all types actually comes from people with well-off backgrounds who tend to be very well-adjusted".