Enemy within: Counterterrorism experts fear sleeper cells could be poised inside US

American counterterrorism agencies are quietly monitoring suspected sleeper cells on U.S. soil in the wake of joint U.S.–Israel strikes on Iran, stepping up surveillance amid heightened fears of possible retaliation from Iran-linked operatives or sympathizers.

Federal and local law enforcement have also boosted on-the-ground security in major U.S. cities as part of a precautionary posture, even though no specific, credible threats have been publicly identified.

The move comes on the heels of a Saturday morning operation, dubbed Operation Epic Fury, that resulted in the death of Iran’s Supreme Leader Ayatollah Ali Khamenei and several senior Iranian officials in a coordinated U.S.–Israeli military campaign.

Security experts warn that the fallout could extend beyond the Middle East.

‘If ever there’s going to be a Hezbollah cell or a Hamas cell act in the United States in a violent way it’s now,’ Chris Swecker, a former assistant FBI director, told Fox News.

‘Both organizations are Iranian-backed all the way. Both organizations have had a presence in the United States since the 1980s,’ he added.

Swecker said U.S. authorities have long been aware of domestic networks and sympathizers.

‘We know that they have cells here. We also know that there are lone sympathizers, many of whom have come out in these protest groups,’ he added.

He also warned that border security vulnerabilities may have compounded the risk.

‘We just come off four years of open borders, and I have said before that that was an open door for terrorists to terrorist cells and terror sympathizers to infiltrate. Many were already here, but it’s impossible that they would not infiltrate into these particular groups and sort of act as catalysts, as enablers.’

Against that backdrop, the FBI is moving to reinforce its domestic security posture.

FBI Director Kash Patel said Saturday that counterterrorism and intelligence teams are now on high alert amid ongoing U.S. actions involving Iran.

‘Last night, I instructed our Counterterrorism and intelligence teams to be on high alert and mobilize all assisting security assets needed,’ Patel wrote on X.

Patel added that while the U.S. military is handling force protection overseas, the FBI ‘remains at the forefront of deterring attacks here at home’ and will continue working around the clock to protect Americans.

Jason Pack, a retired FBI supervisory special agent and Fox News contributor, said heightened monitoring is routine when U.S. military operations intersect with adversaries that have historically responded through indirect or unconventional means.

‘The intelligence and counterterrorism communities work on this kind of scenario continuously, long before any conflict begins,’ Pack said. ‘When the United States commits to a joint military campaign with Israel, the domestic threat environment doesn’t simply remain static. It could shift, potentially significantly.’

He noted that adversarial actors — including Hezbollah, Hamas’s external networks and Iranian Revolutionary Guard Corps proxies — have historically demonstrated both intent and, in some cases, the capability to retaliate against U.S. military actions.

Meanwhile, Homeland Security Secretary Kristi Noem said she is ‘in direct coordination with our federal intelligence and law enforcement partners as we continue to closely monitor and thwart any potential threats to the homeland.’

The heightened alert comes as parts of the Department of Homeland Security face a partial shutdown, raising additional questions about resources and operational strain at a time when federal agencies are on elevated watch.

While no specific plots have been identified, officials acknowledge the threat environment could shift quickly as tensions overseas evolve and whether that escalation reaches American soil remains to be seen.

Fox News Digital’s Michael Dorgan and Kelley Kramer contributed to this report.