Although federal law enforcement has arrested numerous people in recent years associated with or inspired by Sunni Muslim extremist groups like al Qaeda and ISIS, the FBI has also disrupted the operational activity of a small number of individuals linked to the Shiite Muslim Hezbollah.

US counterterrorism officials have not publicly released estimates of how many Hezbollah members are suspected of being in the country, but a senior US national security official tells CNN that the government continues to investigate and degrade Hezbollah's capabilities in the United States.

"Hezbollah is certainly a national security priority, because they have the capacity to commit hostile acts but have not necessarily had the motive to do so," the senior US official said. "Our efforts have been to reduce their capacity in case their motives shift."

Information documented in a criminal complaint listed Kourani as a self-described "sleeper" who, along with another arrested associate, was supposed to "maintain ostensibly normal lives but could be activated and tasked" to conduct operations.

As the Trump administration gears up for possible military action with Iran, it is unclear how extensive the role of federal law enforcement has been in war-planning efforts. An FBI spokesperson declined to comment on what, if any, part the bureau is playing in White House meetings about Iran.

"We know that Iran has used proxies, to include Hezbollah, around the world for its malign activity," said Lisa Monaco, former White House homeland security adviser to President Barack Obama and assistant attorney general for national security. "As tensions escalate, it is important that there be a coordinated approach across the counterterrorism community, so that the FBI and others with responsibility for security here at home are aware of US moves and potential responses to those moves."

Although federal law enforcement has not traditionally been party to decision-making discussions about military action abroad, the FBI would be responsible for detecting any retaliatory threats inside the country that come in response to armed conflict between Iran and the United States.

One former counterterrorism official notes that Hezbollah's experience and training pose a challenge to efforts by law enforcement to understand the group's motives.

"Unlike those linked to ISIS, Hezbollah operatives are highly trained and incredibly disciplined," said Robert Baer, a retired CIA operations officer and CNN national security analyst. "You're not going to find them getting caught up in the net of law enforcement by boasting online about wanting to conduct attacks."