How an Alert Passport Office Worker Helped Unravel an Alleged ISIS Pipeline in America “That passport official played an important role," Justice Dept. official says.

 -- It wasn’t an NSA surveillance program or even some overseas CIA source that launched the FBI on a path to unraveling the largest group of alleged ISIS recruits in America. Instead, it was an alert and quick-thinking passport specialist, according to federal law enforcement officials and details outlined in court documents.

Last year, when the FBI office in Minneapolis received a call from the city’s passport office, U.S. counterterrorism officials had never heard of Abdullahi Yusuf -- the 18-year-old college student who allegedly tried to join ISIS one month later and whose friends are now at the center of a high-stakes terrorism investigation spanning two continents.

“It’s a guy, just a friend,” he told the specialist, according to court documents.

But when the specialist pressed him for more information about his “friend” in Istanbul and his plans while there, Yusuf couldn’t offer any details, the documents allege.

“[He] became visibly nervous, more soft-spoken, and began to avoid eye contact,” the documents say. “Yusuf did not appear excited or happy to be traveling to Turkey for vacation.”

In fact, the passport specialist “found his interaction with Yusuf so unusual that he contacted his supervisor who, in turn, alerted the FBI to Yusuf’s travel,” according to the court documents.

Within days, the FBI placed Yusuf under surveillance, according to the documents. And a month later, FBI agents watched as his father dropped him off at a local mosque, and then Yusuf worked his way to Minneapolis-St. Paul International Airport. Apparent destination: Syria.

But FBI agents stopped him from leaving the country, and further investigation tied him to a 20-year-old named Abdi Nur, according to the documents. The next day, however, Nur successfully slipped out of the country before the FBI could stop him, too.

“Nur unfortunately made it to Syria, where today he is an active recruiter for ISI[S],” U.S. Attorney Andrew Luger said at a news conference Monday morning. “He is in regular contact with his friends, he advises them, and he serves as a source of inspiration to those who want to replicate his success.”

After an FBI investigation that began more than 10 months ago, six of those friends were arrested in the United States on Sunday, accused of trying to join ISIS. Yusuf and another friend were arrested late last year.

And it all began with a skeptical passport office worker.

“That passport official played an important role” in taking down the ISIS recruitment pipeline in Minneapolis, one Justice Department official said.

Yusuf has since pleaded guilty to conspiring to support ISIS. While he awaits sentencing, he has been living in a halfway house in Minneapolis and regularly meeting with a local group that hopes to “foster his connections to his future and his community.”

His attorney has argued Yusuf planned to join ISIS before it began beheading Americans and others. In addition, ISIS had yet to be designated a foreign terrorist organization by the U.S. government when Yusuf applied for his passport and began taking steps to go to Syria.

But prosecutors have insisted the group's brutal ways and anti-American sentiments were still well-established at the time.