What is a Sleeper Cell? US warns citizens that risk 'has never been higher'

Iran has warned that there will be “everlasting consequences” following the US attack - (AFP via Getty Images)

The US Customs and Border Protection (CBP) said on Monday that the risk of sleeper cells in the US has "never been higher,” following the US strike on three nuclear sites in Iran.

The US Air Force deployed B-2 stealth bombers armed with GBU-57F/B Massive Ordnance Penetrator bombs against Iran’s nuclear facilities on Sunday morning in an operation named “Midnight Hammer.”