Douglas McAuthur McCain, the first known American to be killed while fighting alongside Islamic militants in Syria, was an undistinguished 33-year-old raised in Minnesota who most recently worked as a caregiver in California.

So what compelled him to leave for the Middle East this spring and to take up arms on behalf of religious extremists?

The U.S. National Security Council confirmed McCain's death on Tuesday. The State Department said U.S. officials had been in contact with McCain's family.

The issue of Americans joining radical forces in places like Syria, getting training and even indoctrination in terrorist ideology, has pushed to the forefront concern among U.S. officials, who fear one or more might try to return and commit terrorist acts on American soil.

McCain had reportedly made his way from the United States to Turkey, and then into territory controlled by the Islamic State, the radical organization that has swept through northern Iraq terrorizing many with its brutal version of Islamic law. Over the weekend, McCain took part in an attack on a Syrian opposition checkpoint near Aleppo, according to NBC News, which first reported the story.

Rebels in the Free Syrian Army, one of groups fighting the regime of Syrian leader Bashar al-Assad, retaliated, killing McCain. They beheaded six other Islamic State fighters, but not McCain, and posted photos on Facebook, NBC reported, attributing the information to the rebel army. The rebels reportedly found McCain with his U.S. passport and $800 in his pocket.

The FBI also is investigating. The bureau's field office in Minneapolis for almost a decade has looked into the cases of several young Somali-Americans joining the terrorist group al-Shabab in Somalia.

E.K. Wilson, spokesman for the field office, told The Associated Press: "We have done extensive outreach recently, as we have the last seven years, but we've had a concerted effort ... over the last few months" involving travel to Syria.

U.S. officials said McCain's posts to Facebook, Twitter and other social media had put him on a watch list for international flights.

"We continue to use every tool we possess to disrupt and dissuade individuals from traveling abroad for violent jihad and to track and engage those who return," said Caitlin Hayden, a spokeswoman for the National Security Council. 

Thousands of foreign fighters have flooded into Syria in recent years, joining rebel groups, of which many are far more radical than the Free Syria Army. Most of the fighters are Europeans. The FBI and other U.S. officials estimate anywhere from several dozen to more than 100 Americans have gone to fight in Syria.

Australian intelligence chief David Irvine said Wednesday that 15 Australians are believed to have died fighting in Syria and Iraq, and that about 60 Australians are fighting with jihadist groups such as IS.  

The U.S. government's "largest concern is the regional and even global aspirations" of the Islamic State," said Pentagon spokesman Rear Adm. John Kirby.

"Obviously, we’re concerned about Americans that become attracted," Kirby said. If they join forces with a terrorist group, they become enemies of the state, he said: "They take on those actions at their own peril."

McCain was born in Illinois and raised in the Minneapolis suburb of New Hope, where he attended Robbinsdale Cooper High School. He ran into some trouble with the law, with convictions for theft, drug possession, disorderly conduct and driving after his license had been revoked, the Minneapolis Star Tribune reported.    

Later, he moved to San Diego, California, as did his mother and a sister. McCain worked for a Somali-operated African Spice market, now closed, and attended a local community college. He also had a daughter, almost 1 year old, family members told the Star Tribune.

According to McCain's social media accounts, which were taken down Tuesday, he converted from Christianity to Islam in 2004.

"I will never look back the best thing that ever happened to me," he tweeted in May. He identified himself on Facebook as "Duale ThaslaveofAllah" and on Twitter as Duale Khalid, "iamthetooth."

A selection of Twitter posts, reported by the San Diego Union-Tribune, reflect McCain's changing attitudes and circumstances.

In December 2012, he tweeted that the film "The Help"– about black maids working for white families in Mississippi in the 1960s – was "starting to make me hate white people."

Along with racist and sexist views, the posts show McCain's enthusiasm for basketball, rap music and, especially, his faith.

"To all my Muslim out there stand strong we will soon be 1... In sha Allah," he tweeted in May.

McCain’s cousin, Kenyata McCain, described him as a "humble, caring man" who "lost his identity" after becoming involved with Somali Muslims in the Minneapolis area. Minnesota has the country's largest community of Somalis, with an estimated 32,000 people of that ancestry. 

"I know that he had strong Muslim beliefs," the cousin told the Star Tribune, "but I didn't know that he was in support of ISIS [an earlier acronym for the Islamic State]. I didn't think he would be." 

Minnesota Public Radio also reported that, from McCain's Facebook page, it appears he knew Abdirahman Muhumed, "a Minneapolis man who went to Syria and joined the Islamic State."

Muhumed had posted a photo of himself holding a rifle and a Qur'an, eliciting negative responses from Facebook "friends," MPR said. But McCain, in a Feb. 19 post, encouraged Muhumed to "continue protecting our brothers and sisters."

Kenyata McCain said she was in regular contact with her cousin and exchanged messages with him as recently as last Friday. "He was telling all of us he was in Turkey," she told the Star Tribune.

State Department spokeswoman Jen Psaki said the threat of terrorism by people training with religious extremists is a priority for policymakers in the U.S. and other Western nations.

"The issue of foreign fighters and the concern of individuals with Western passports or passports that would enable them to travel into countries where they can do harm is certainly at the top of our agenda and the top of the agenda of many countries," she said at a briefing Tuesday.

Last month, Attorney General Eric Holder said he was concerned fighters from Europe and the United States were supporting violent insurgents in Syria and joining forces with Yemeni bomb makers.

In July, FBI Director James B. Comey addressed an international law enforcement conference in Miami, saying he was "especially concerned about Syria."

"Syria serves as a breeding ground, a training ground, and a networking ground for thousands of jihadis all over the world," he said. "They have gone there in huge numbers to join the fight with groups like al Nusra or ISIL. The going is very worrisome. It is the coming out that worries me even more.

"We’re trying to build effective relationships with the private sector and our government partners," Comey said. "We are trying to train so we learn to play well. We are engaged in simulations that as best we can are intended to duplicate what we face in real life."

Jonathan Adelman, associate professor at the University of Denver’s Korbel School of International Relations, said foreign fighters pose a serious threat to Western nations, including the United States.

He estimated roughly 100 Americans, between 400 to 500 British citizens and several hundred French are among the 2,000 Westerners fighting on behalf of the Islamic State.

"I think this is something that really we have to take very seriously," Adelman said. The threat "isn’t as remote as we thought it was after Osama bin Laden was killed."

Many of these foreign fighters are being recruited through social media, he said.

"I think for a lot of these kids ... there's a level of excitement about this. 'We’re going to have foreign adventure. We are going to stand up against all the evils of this world,'" Adelman said. "But, it’s frightening. We’re a country of 315 million people. All it’s going to take is a dozen of these people, with the fighting experience they’re getting in Syria and Iraq, and all the training they’re getting, to be able to come in here quite legally, and we’re fairly vulnerable."

Three types of people are most susceptible to involvement with terrorist organizations, according to Clifford May, a national security specialist and president of the Washington-based Foundation for the Defense of Democracy: sociopaths, those searching for meaning, and highly educated people with a misguided sense of mission.

The last category, he said, includes the Islamic State’s leader, Abu Bakr al-Baghdadi. He claims to hold a doctorate from Islamic University in Baghdad and degrees in Islamic studies and history. Al-Baghdadi, for whom the U.S. has offered a $10 million bounty, "believes that Americans don’t deserve the power they wield" and is willing to take it by force, May said.

May speculated McCain might be one of "what you might call ‘lost boys’ who are desperate for meaning and a transcendent cause."

Journalist released 

The renewed concern over foreign fighters came as American journalist Peter Theo Curtis returned to the United States late Tuesday, just two days after being freed from nearly two years captivity at the hands of the Islamist Jabhat al-Nusrah group in Syria. 

In a statement, Curtis thanked U.S. officials and the Qatari government for intervening on his behalf.