What is a sleeper cell, and why are US officials on high alert?

(NewsNation) — As tensions escalate between the United States and Iran, the threat of sleeper cells has "never been higher," according to U.S. Customs and Border Protection, although no specific or credible threats have yet emerged.

In a memo obtained by NewsNation, CBP Commissioner Rodney Scott said, "Thousands of Iranian nationals have been documented entering the United States illegally and countless more were likely in the known and unknown got-a-ways.”

While there is no current actionable intelligence, Scott emphasized the need for vigilance: “Though we have not received any specific credible threats … the threat of sleeper cells or sympathizers acting on their own, or at the behest of Iran has never been higher."

What are sleeper cells, and how do they operate?

Sleeper cells are spies or terrorists who operate in rival countries and remain inactive until they are essentially ordered to act.

NewsNation national security contributor Tracy Walder explained that while the concept of Iranian sleeper cells in the U.S. is not new, there may have been a recent influx of agents.

“They have sleeper cells here. Quite frankly, they have for decades. … They are just waiting for the right moment," Walder said.

A recent Homeland Security threat assessment confirms that Iran remains a primary state sponsor of terrorism and continues efforts to target both current and former U.S. officials. That assessment echoes similar warnings issued by the Biden administration in 2024.

How do sleeper cells recruit and train?

Recruitment is typically done through networks, religious institutions or online platforms like social media. Recruiters typically appeal to emotions, such as alienation, injustice or religious duty.

Retired FBI Supervisory Special Agent James Gagliano underscored the long-term nature of the threat, referencing the 2020 U.S. airstrike that killed Iranian General Qassem Soleimani.

"Iran has basically been on a warpath for us," he said. "The sleeper cells are a real problem. ... It's people leaking across ... from Iran, China, North Korea, Russia that might stay dormant until they're either inspired or, in this case, with the supreme leader in Iran directed to cause some type of mayhem or harm."

What have past attacks taught us?

Identifying sleeper cells is extremely difficult. Walter pointed out that some individuals may be homegrown or enter the U.S. undetected.

She noted that even when they arrive legally, they "lead these seemingly normal lives," and don't draw any attention to themselves until it's too late.

"If you go back as far as to September 11, those individuals had actually entered the US legally," Walder said. "Had we checked their visas properly, obviously we would have known, because we know that now, but that's the difficult part of all of this."

"They have to get messages in some way. They have to get a signal in some way, and that's where I'm hoping they will slip up and we will ultimately catch them," she continued.

How can the public stay informed and vigilant?

Following the recent U.S. airstrikes on Iranian nuclear sites, the State Department issued a “worldwide caution” security alert warning of heightened risks.

Similarly, the Department of Homeland Security issued a National Terrorism Advisory System Bulletin, advising vigilance without encouraging fear.

Officials urge the public to monitor official alerts from DHS, local law enforcement and the FBI and to report suspicious activity through the "If You See Something, Say Something" campaign.

Rich McHugh, Diana Falzone and Patrick Djordjevic contributed to this report.