What are sleeper cells and why are the FBI on alert for them after Trump’s strikes on Iran?

The United States has ramped up its monitoring of Iranian sleeper cells as President Donald Trump’s strikes on three major Iranian nuclear sites decisively propelled the U.S. into Israel’s war.

Following Saturday’s strikes, which Trump claimed “totally obliterated” Iran’s nuclear sites of Fordow, Natanz and Isfahan, both White House and FBI officials have been on high alert for Iranian sleeper cells.

Sleeper cells comprised of spies or terrorists hiding out in the U.S. or Western countries, remain inactive, often living quiet and unassuming lives working regular jobs until they are ordered to act on a mission. (Think Philip and Elizabeth Jennings, the fictional Cold War-era KGB spies who pose as the typical American couple with kids in suburban DC in FX’s series The Americans.)