A federal alert sent to law enforcement agencies across the United States has warned that encrypted communications believed to originate in Iran may be linked to potential “sleeper assets” operating outside the country.

Authorities say the signal was intercepted shortly after the reported death of Ali Khamenei, who was killed in a US-Israeli attack on February 28. The message, according to preliminary signals analysis cited by ABC News, appears to have been transmitted across several countries and encoded for recipients who possess a matching encryption key.

Security agencies say the broadcast may have been designed to relay instructions to covert operatives without using internet or cellular networks.

Encrypted transmission prompts heightened monitoring

The federal advisory described the signal as a newly emerged broadcast station with “international rebroadcast characteristics”. Investigators believe the transmission was intended for clandestine recipients capable of decoding it.

Officials say the exact contents of the message remain unknown. However, the alert warned that the sudden appearance of such a station requires heightened situational awareness among authorities.

Law enforcement agencies were instructed to increase monitoring of suspicious radio-frequency activity while intelligence services assess whether the broadcast was linked to operational directives.

The advisory emphasised that no specific operational threat or target had been identified.

The intercept has nevertheless raised concerns among US and allied security agencies as the conflict involving Iran entered its second week. With leadership changes under way in Tehran following Khamenei’s death, officials say it remains unclear whether sleeper assets abroad could be activated.

Wider security concerns and related incidents

Earlier warnings about a heightened threat environment had already been issued by the Department of Homeland Security after strikes on Iran’s nuclear facilities in June 2025. The advisory at the time urged vigilance against violent extremists who could act independently in response to the conflict.

Political and security figures have also raised concerns about possible infiltration. Ted Cruz said authorities across the country were maintaining vigilance against sleeper cells, while former Department of Homeland Security adviser Charles Marino said intelligence officials could not definitively assess the scale of infiltration inside the United States.

Separate incidents have heightened fears. On March 1, a gunman with pro-Iranian regime sympathies opened fire at a bar in Austin, killing two people and injuring 14 before police shot him. Investigators also arrested two teenagers in New York City after an explosive device was thrown at an anti-Islam protest. Authorities say the suspects are being investigated for possible links to ISIS.

Meanwhile, military tensions remain high. Donald Trump has indicated further strikes on Iran could follow, and three B‑52 Stratofortress aircraft capable of carrying large weapons loads have landed at RAF Fairford after the UK government led by Keir Starmer allowed American use of British air bases.

Authorities say monitoring efforts will continue as investigators determine whether the intercepted transmission carries operational instructions.