LONDON -- The sixteen-year-old twins slipped out of their beds in the middle of the night, grabbing passports and a few possessions. With that they were gone, hopping a flight from Britain to Turkey and sparking fears they were lost to a troubling and growing sisterhood: jihadi brides.

Since the civil war in Syria erupted in 2011, numerous reports have surfaced of Western women traveling to marry Islamist fighters. Two Somali sisters from Norway reportedly took the same route as the teens who disappeared from England's northern city of Manchester this week, flying to Turkey and disappearing along the border with Syria.

The exact number of women who have gone to marry jihadis is impossible to ascertain – which is exactly why experts are worried.

“It’s a phenomenon that’s been going on for awhile,” said Haras Rafiq, an expert on extremism with the London-based Quilliam Foundation. “It’s been going on since the beginning of the conflict and before that in Afghanistan and other places.”

The recent declaration by the Islamic State of Iraq and al-Sham (ISIS) that it has established a caliphate in Iraq and Syria is “an interesting development” that could affect the flow, he added.

After ISIS' announcement of an Islamic state, the Sunni extremist group put out the call for doctors and others to help set up infrastructure.

“Of course when building a state, what better way to build a state for longevity than to have families,” Rafiq noted. “To have families you need women to come over as well.”

Whereas some women previously had no desire to get drawn into a fight, “psychologically some of them who believe in this particular theology and ideology now will feel it’s incumbent on them to go – and also that it’s not just to go and fight and satisfy the needs of jihadi soldiers.”

"They tend to be extremely pious and have been ISIS ‘fangirls’ for the duration of the Syrian conflict, active on social media"

Analysts are quick to point out that Western women turning up in Syria have not just been going to marry – many have turned up fighting on the front lines, too.

But just last week, Greater Manchester Police Chief Constable Peter Fahy warned young women against becoming brainwashed by "perverted messages" and traveling to the Mideast to become "jihad brides."

Melanie Smith, a research associate with the International Centre for the Study of Radicalisation, has been tracking the movements of foreign fighters to Syria. They include 30 women who have traveled from Europe to the country.

She said that most are undertaking “typical” female roles: making house and establishing community and support structures – not fighting. Most of the women fit into two groups, she said: those who travel with their husbands to jihad, and those who travel to Syria or Iraq to get married.

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The latter is “certainly a trend,” she said. “Many questions flood the male foreign fighters on social media about the possibility of marrying them or even being their third wife. They are generally answered in a positive way and asked to move to a more private platform.”

Like others, Smith said there is no concrete way to gauge the numbers of women involved – just as there’s no hard way to gauge the number of male foreign fighters.

“However, I would assume that this is a trend that will grow,” she said.

Nailing down the profile of a potential jihadi bride is equally difficult – though most tend to be young.

“They tend to be extremely pious and have been ISIS ‘fangirls’ for the duration of the Syrian conflict, active on social media,” she added.

Quilliam's Rafiq said the profile is too broad to nail down: it could girls fighting with their parents, experiencing the identity crises that often accompany teen years, or “it could be anything.”

“This could be girls who are looking for an avenue, something to do for themselves – some sort of empowerment. Charismatic recruiters either online or on the ground will play on this,” he said, noting that most teens – not just Muslims – at some point question their identity.

The recruiters will play up the “romantic, idyllic” notion of an Islamic state when encouraging women to come, Rafiq said, careful to note that “this isn’t just a Muslim phenomenon.”

One thing experts agree on is that the role of social media cannot be overstated.

“There are people who are enticing women to come over social media, romanticizing the whole aspect of coming over and imploring them that it’s their religious duty,” Rafiq said. “If a particular Twitter account or Facebook account is closed down, another one pops up.”

"Some will feel that it’s their religious duty to go – so it doesn’t actually matter that they find the husband before they go over"

There are numerous reports of women going to Turkey with the aim of entering Syria – but analysts wonder how many others have gone in through other entry points and were not reported by their families.

“The question really is: how many are out there already that we don’t know about, and how many will come back?” Rafiq said. “There isn’t as much a focus on women as there is on male jihadi soldiers. This is something we should be paying more attention to.”

While some of the women are set up with a specific groom, others go unattached and open-minded, spurred on by their faith and hoping to find their husbands on the ground.

“Some will feel that it’s their religious duty to go – so it doesn’t actually matter that they find the husband before they go over,” Rafiq said.

That could be dangerous, according to Stig Hansen, an expert on security and political Islam based at the Norwegian University of Life Sciences.

“You don’t know what kind of reception you will have or anything like that,” Hansen explained. “You can be received as captured by a group and used for other purposes or blackmail.”

Hansen has been studying the cases of at least four Norwegian women of Somali origin who have heeded the call.

“They seem influenced by a quite drastic dose of teenage romanticism as well as ideology,” Hansen said. “They do not really know what they are going to. Ignorance about Islam and ignorance about various conflicts around the world is an advantage – then it becomes easier to simplify the conflict.”

Many of the parents who have sounded alarm say their daughters lacked the funds to make a trip to Syria, raising questions over how they made their way to the fight. Experts say networks likely exist to help fund and facilitate the journeys, from paying for airline tickets to introductions on the ground.

While the Internet plays a large role, Hansen said some women could be encouraged by local sheiks or imams or family connections to fighters recently returned from Syria with connections on the ground.

“You can either get in touch with them on the internet, you can get in touch with a gatekeeper at the local mosque of some sort or in your family or you can be in touch with nobody and go on your own,” Hansen said. “Usually, you get something on the Internet – are inspired by something on the Internet.”

Attention to the phenomenon in the media and halls of government could serve only to fan the fires, Hansen added.

“People read about the jihadi brides going there and it inspires new people to go there.”