What is a sleeper cell - as Iran 'threatened Trump with revenge terrorist attacks inside US' days before nuclear strikes

Trump received a communiqué from the regime just days before he ordered US military strikes on its nuclear facilities, sources told NBC News. The official message was delivered to Trump through an intermediary at the G7 summit in Canada last week. Sleeper cells are spies or terrorists who operate in the U.S. and Western countries and remain inactive until they are essentially ordered to act.

The president left early on June 16 to consider his options amid the conflict between Israel and Iran, according to the sources. After his departure from the G7 summit, Trump told reporters aboard Air Force One he planned to hold 'early' meetings with his security team in the White House Situation Room after issuing a stunning call for people to 'evacuate' Tehran amid Israeli bomb attacks there.

After the US deployed B-2 bombers to drop more than a dozen 'bunker busters,' Iran's foreign minister called the strikes 'outrageous' and said that Tehran 'reserves all options' to retaliate. The Department of Homeland Security issued a national terrorism bulletin Sunday warning of possible cyber attacks and violence, including antisemitic hate crimes, following the strikes.

Although there are 'no specific credible threats,' the department warned that low-level cyber attacks against US networks are likely. Cities across the United States are on high alert following the airstrikes on three nuclear sites in Iran. Police in New York City and Washington DC revealed they increasing their presence amid fears of a retaliatory attack.