One day this April, instead of coming home from school, two teenagers left their valley high in the Caucasus, and went off to war.

In Minneapolis, Minnesota, a 20-year-old stole her friend's passport to make the same hazardous journey.

From New Zealand, came a former security guard; from Canada, a hockey fan who loved to fish and hunt.

And there have been many, many more: between 16,000 and 17,000, according to one independent Western estimate, men and a small number of women from 90 countries or more who have streamed to Syria and Iraq to wage Muslim holy war for the Islamic State.

Abu Bakr Al-Baghdadi, the group's leader, has appealed to Muslims throughout the world to move to lands under its control ' to fight, but also to work as administrators, doctors, judges, engineers and scholars, and to marry, put down roots and start families.

"Every person can contribute something to the Islamic State," a Canadian enlistee in Islamic State, Andre Poulin, says in a videotaped statement that has been used for online recruitment. "You can easily earn yourself a higher station with God almighty for the next life by sacrificing just a small bit of this worldly life."

The contingent of foreigners who have taken up arms on behalf of Islamic State during the past 3 1/2 years is more than twice as big as the French Foreign Legion. The conflict in Syria and Iraq has now drawn more volunteer fighters than past Islamist causes in Afghanistan and the former Yugoslavia ' and an estimated eight out of 10 enlistees have joined Islamic State.

They have been there for defeats and victories. Following major losses in both Syria and Iraq, the fighters of Islamic State appear to have gotten a second wind in recent days, capturing Ramadi, capital of Iraq's largest Sunni province, and advancing in central Syria to the outskirts of the ancient city of Palmyra, famous for its 2,000-year-old ruins.

There are battle-hardened Bosnians and Chechens, prized for their experience and elan under fire. There are religious zealots untested in combat but eager to die

They include around 3,300 Western Europeans and 100 or so Americans, according to the International Center for the Study of Radicalization, a think tank at King's College London.

Ten to 15 percent of the enlistees are believed to have died in action. Hundreds of others have survived and gone home; their governments now worry about the consequences.

"We all share the concern that fighters will attempt to return to their home countries or regions, and look to participate in or support terrorism and the radicalization to violence," Nicholas J. Rasmussen, director of the U.S. government's National Counterterrorism Center, told a Senate hearing earlier this year.

"Just like Osama bin Laden started his career in international terrorism as a foreign fighter in Afghanistan in the 1980s, the next generation of Osama bin Ladens are currently starting theirs in Syria and Iraq," ICSR director Peter Neumann told a White House summit on combating extremist violence in February.

One problem in choking off the flow of recruits has been the variety of their profiles and motives.

Associated Press reporters on five continents tracked some of those who have left to join Islamic State, and found people born into the Islamic faith as well as converts, adventurers, educated professionals and people struggling to cope with disappointing lives.

"There is no typical profile," according to a study by German security authorities, obtained by AP.

The study reported that among people leaving that country for Syria out of "Islamic extremist motives," 65 percent were believed to have prior criminal records. They ranged in age between 15 and 63. Sixty-one percent were German-born, and there were nine men for every woman.

In contrast, John G. Horgan, a psychologist who directs the Center for Terrorism & Security Studies at the University of Massachusetts Lowell, found some common traits among American recruits or would-be recruits for jihad. Typically, he said, they are in their late teens or early 20s, though a few have been in their mid-30s.

"From a psychological perspective, many of them are at a stage in their lives where they are trying to find their place in the world ' who they are, what their purpose is," Horgan said. "They certainly describe themselves as people who are struggling with conflict. They are trying to reconcile this dual identity of being a Muslim and being a Westerner, or being an American."

Some are driven by religious zeal to protect the caliphate, or Muslim theocracy, that the Islamic State has proclaimed in the one-third of Syrian and Iraqi territory now in its hands; others are thrilled by the chance to join what is tantamount to a secret and forbidden club.

Still others appear to enlist mainly because others do.

"What they have in common is that they are young, they are impressionable and they are hungry for excitement," Horgan said.

Once recruits arrive in areas held by Islamic State, they appear to receive only rudimentary military training ' including how to load and fire a Kalashnikov assault rifle. Nonetheless, they have been involved in "some of the most violent forms of attacks" by the group, including suicide bombings and filmed beheadings of foreigners, said William Braniff, executive director for the National Consortium for the Study of Terrorism and Responses to Terrorism, a multidisciplinary research center headquartered at the University of Maryland.

Areeb Majeed, 23, from a suburb of Mumbai, India, joined Islamic State in May 2014 and fought for six months, killing up to 55 people and taking a gunshot to the chest.

But all was not heroics. He eventually called his parents from Turkey and asked to come home, according to Indian newspapers. Majeed's chief complaint, officials from India's National Investigation Agency were quoted as saying, was that the group didn't pay him, and made him clean toilets and haul water on the battlefield.

Often, though, the foreign combatants use social media to serve as "role models and facilitators for the next volunteers," Braniff said.

"Before I came here to Syria, I had money, I had a family, I had good friends, it wasn't like I was some anarchist or somebody who just wants to destroy the world, to kill everybody," said Poulin, the Canadian ISIS recruiter.

"Put God almighty before your family, put it before yourself, put it before everything. Put Allah before everything," the bearded and bespectacled transplant from Ontario urges in the video.

Poulin's jihad ended last August; he was reported killed during an assault on a government-controlled airfield in northern Syria.

But not, according to the Canadian Broadcasting Corp., before he had recruited five others from Toronto to come fight for the Islamic State.(+++)