AQSA Mahmood's family saw her as an intelligent and popular teenager who helped care for her three younger siblings and her grandparents at their home in Glasgow, Scotland. She listened to Coldplay, read Harry Potter novels and drank Irn Bru, a Scottish soft drink.

She aspired to be a pharmacist or a doctor, and they did not expect her to leave home in November 2013 to go to Syria, where the authorities say she is one of the most active recruiters of young British women to join the Islamic State in Iraq and Syria (ISIS).

The authorities are investigating possible links between Mahmood, who goes by the name Umm Layth (meaning Mother of the Lion), and the disappearance last week of three teenagers from London. They, too, are believed to have travelled to Syria to join the ISIS terrorist group.

The apparent trend of studious, seemingly driven young women leaving home to join violent militants has become disturbingly familiar.

A London Metropolitan Police official on Monday said one of the girls, Shamima Begum, sent a Twitter message to a woman on Feb 15, a couple of days before they left Britain, but declined to disclose her name.

Experts who track Islamist militant activity online, including Ms Audrey Alexander at the International Centre for the Study of Radicalisation, in London, have identified that woman as Mahmood, 20.

She is thought to be living in Raqqah, Syria, the de facto capital of ISIS, and married to an Islamist militant as well as acting as a virtual den mother offering sometimes stern advice to her peers following in her footsteps.

As the families of the three missing girls made tearful appeals for them to return home, Mahmood's family also issued a statement last weekend addressed to their daughter, whom they called a "disgrace". They said they were "full of horror and anger" that she "may have had a role to play" in recruiting the girls for ISIS.

"Your actions are a perverted and evil distortion of Islam," the family said in their statement, released through their lawyer Aamer Anwar. "You are killing your family every day with your actions. They are begging you to stop if you ever loved them."

The young women - Kadiza Sultana, 16; Shamima Begum, 15; and Amira Abase, 15 - have been described by a classmate as studious, argumentative and driven, not unlike Mahmood. Sultana's Twitter feed shows that she followed many accounts of Islamist fighters.

Begum, who sent a Twitter message to Mahmood, asked her own followers before she left to "keep me in your duas", or prayers. (Their accounts were recently disabled.)

The teenagers told their families on Feb 17 that they would be out for the day, but security camera video at Gatwick Airport, near London, shows they had boarded a Turkish Airlines flight to Istanbul, and the Metropolitan Police on Tuesday said they had arrived in Syria.

Another classmate from the girls' school, Bethnal Green Academy in east London, took a Turkish Airlines flight last December and is thought to be in Syria. Police at the time questioned the three teenagers over their classmate's disappearance, said a Metropolitan Police officer.

Like the three girls, "Aqsa was very intelligent, very liked, very bubbly, kind, caring", said Mr Anwar, the lawyer. But ISIS has turned her into "a poster girl in Britain for recruitment", he said, "and she herself is a high-value recruit".

Members of Mahmood's family said they had "absolutely no inkling" of her radicalisation, according to Mr Anwar. The oldest of two sisters and a brother, she lived with her parents and grandparents in a middle-class area of Glasgow.

None of the women in her family wore a head scarf, Mr Anwar said, but one day, Mahmood began wearing a hijab and became "increasingly vocal and angry" about events in Syria.

"But you can go to any Muslim household," he added, "and you would hear similar arguments being made."

"It is those young people who are liked, who are smart, who think, who are caring, who are ripe for radicalisation," he said, not the outcasts.

When they do not receive adequate answers from their families, schools or the local mosque, they often turn to the Internet, Mr Anwar said.

Last year, a pair of 17-year-old twins from Manchester who travelled to Syria attracted widespread attention because they had been straight-A students who wanted careers in medicine.

The precise role Mahmood might have played in the flight of the three London teenagers is unclear since it was Begum, not Mahmood, who initiated their exchange on Twitter.

Their conversation quickly moved to an encrypted social media channel, which is standard practice among would-be Islamist militants seeking practical information on how to reach Syria.

There are about 100 British women among the 550 Western women who are thought to have joined Islamist groups in Syria and Iraq, according to the Soufan Group, a security consultancy based in New York.

Female recruits are generally younger than their male counterparts, said Mr Ross Frenett of the Institute for Strategic Dialogue, a London research organisation that studies extremism.

As the radicals see it, "a 15-year-old makes a good wife", he said. "A 14-year-old male is less useful as a combatant."

Women who join ISIS try to entice other women to marry militants and help them build a new, retrograde Islamic society.

Mahmood has emerged as one of ISIS' most vocal supporters and also one of its most established online recruiters, according to the SITE Intelligence Group, which monitors Islamist militants' online activity.

She has tried to incite terrorist attacks on Western countries through her Tumblr blog and multiple Twitter accounts, calling on British Muslims to follow the example of "brothers from Woolwich, Texas and Boston". But she has also occasionally alluded to boredom with her housewifely duties.

She has tweeted that, compared with Scotland, the winters in Syria "are too much", and offered this advice: "Sisters, please don't forget to pack thermal clothing or you'll regret it later on."

She also admonished them: "Sisters, please for the sake of Allah contact the sisters whom (sic) are online rather than approaching the brothers. Also know the fact many brothers whom you contact and chat to are married. Have some self-respect and don't be a homewrecker :)"

The families of the three teenagers have criticised security services in Britain for failing to intervene to stop their daughters from going to Syria, even though they were monitoring Mahmood's online activity.

After she left home more than a year ago, Mahmood called her parents from the Turkish border, telling them she would next see them on "Judgment Day" and take them to heaven, holding their hands.

But British security services advised her parents to keep their daughter's disappearance "under the radar", Mr Anwar said.

"Had the security services really been concerned about Aqsa Mahmood's welfare, they would have moved heaven and earth to get her back in November 2013," he said.

NEW YORK TIMES