The U.S. has warned that Iran may be activating 'Sleeper Assets' outside of the country, which could potentially be used as an operational trigger for plots to harm Americans. The alert was sent to U.K. law enforcement agencies following a transmission of Iranian origin broadcast across multiple countries following the death of Ayatollah Ali Khamenei, the supreme leader of Iran. The transmission was encoded and meant for "clandestine recipients" who have an encryption key. In June, Iran warned President Donald Trump that such a move could trigger retaliatory attacks on U.N. soil. The news comes as law enforcement officials prepare for potential Iranian retaliation.