How do American teens end up fighting for ISIS? NBC News offers an interesting look at 15 teens and young adults who have traveled to the battlefield in Syria and Iraq from the US to conduct war and murder for radical Islam. In a segment that aired yesterday, Richard Engel reports on what US authorities — and family members — missed along the way:

American law enforcement officials estimate that roughly 250 Americans have tried to join IS. Most of them never left the United States, raising fears of more homegrown attacks like the one in December in San Bernardino, California. Assistant Attorney General John Carlin, the Justice Department’s top national security officer, told NBC News that his agency has open investigations in all 50 states.

But a few dozen of those American recruits have made the trip to ISIS’s heartland in Syria and Iraq.

In March, NBC News was given a thumb drive by a man claiming to be an ISIS defector. The drive contained the names and snippets of biographical information of more than 4,000 ISIS foreign fighters who entered Syria in 2013 and 2014. The documents, effectively ISIS personnel files, have been verified by West Point’s Combating Terrorism Center and other counterterrorism specialists.

Through the documents, NBC News has identified at least 15 American citizens or residents who have joined ISIS overseas. They fit no particular pattern. Some are from poor Muslim immigrant families. Others had what can be described as privileged backgrounds. Three have Somali backgrounds. One was a Latino convert to Islam. They lived in small towns and cities in New York, Texas, California and places in between.

Engel bases this segment on what appears to be the biggest intelligence find in the fight against ISIS — personnel files smuggled out of Syria by a disgruntled former ISIS bureaucrat on a thumb drive. Engel and his team analyzed the documents and found fourteen of the fifteen American ISIS members featured. The only one not included in the files was the group’s only female, Zakia Nasrin, who married one of the fighters on the list and recruited her brother to join them. Her brother Rasel has since been killed, but Zakia and her husband Jaffrey are reportedly still alive.

What are the common threads? Many of them come from families that legally emigrated to the US, but at least a couple of them came from established American families. Three were converts, and one of them was ironically named after one of America’s greatest military leaders. Douglas McArthur McCain didn’t absorb his namesake’s genius, however, and wound up dead in a skirmish with Syrian rebels. McCain was also one of three with Minnesota ties, although his path to extremism accelerated in San Diego rather than here.

Most of the segment focused on the curious trio of Jaffrey and Zakia Khan and her younger brother Rasel, but perhaps there will be more coming later from Engel on the other shadowy figures who have dedicated their lives and deaths to radical Islam and jihad. PJM’s Patrick Poole calls the featured trio classic examples of the “known wolf” syndrome:

According to NBC News, Jaffrey and Rasel were already known as extremists by the FBI after an informant’s tip. Suspicions were further raised when Jaffrey and Zakia claimed to have “lost” their passports while in Kenya. Rasel admitted to friends that he had been interviewed by the FBI. The report also claims that they were indeed on the terror watch list.

This is a recurring problem I’ve repeatedly identified here at PJ Media as “known wolf” terrorism: again and again, individuals who engage in terrorism or join terrorist groups are already known to law enforcement and national security agencies.

Why were these individuals allowed to slip into Syria?

Further, NBC News reports, curiously, that all three lived in an apartment in Columbus next door to the wife of convicted al-Qaeda cell member Christopher Paul. Paul was known as one of the oldest known American Al-Qaeda operatives …

Poole’s questions should prompt calls for answers — and action. The US and other Western countries had better improve their ability to stop trafficking into Syria, and to come up with ways to spot and confront this radicalization before it steals the souls of teens and young adults in our midst.