The biggest threat is these individuals will become radicalized during their time overseas and return home with the intent of carrying out attacks inside the United States, Mueller said Friday.
"Whether or not they will utilize those associations, utilize that expertise, to undertake an attack upon the homeland" is why U.S. intelligence and federal law enforcement are tracking that flow closely.
Tamerlan Tsarnaev, one of the two brothers who carried out the deadly Boston Marathon bombing earlier this year, was radicalized after fighting alongside Islamic militants in Dagestan, Russia.
In March, Army veteran Eric Harroun was arraigned on federal conspiracy charges after allegedly fighting alongside al Qaeda factions in Syria.
Harroun reportedly traveled to Turkey last November and crossed the border into Syria and linked up with opposition forces led by al Qaeda affiliate Jabhat a- Nusra and members of the Free Syrian Army (FSA) in January, according to court documents.
Once in Syria, Harroun was trained to use rocket-propelled grenades and other weapons and then moved to the front lines of the war by his handlers in the FSA.
On Monday, Sen. Lindsey Graham (R-S.C.) said that the United States should strike an American-born al Qaeda spokesman “without hesitation” after the militant urged attacks against U.S. ambassadors across the globe.
“The use of lethal force against American citizen, al Qaeda leader Adam Gadahn is appropriate and should be utilized without hesitation,” according to Graham.
“Adam Gadahn is an American citizen who has aligned himself with al Qaeda. He should be considered an enemy combatant, not a common criminal,” the South Carolina Republican said.
In May, Attorney General Eric Holder confirmed that the U.S. has killed four American citizens in drone attacks, including Anwar al-Awlaki, an American-born Muslim cleric and his 16-year-old son.