'Persian code, random numbers': Has Iran activated sleeper cells in US? Here's what encrypted radio signals suggest

People carrying placards with images of Iran's new supreme leader Mojtaba Khamenei and late Iranian Supreme Leader Ayatollah Ali Khamenei, during a gathering in Tehran. (Courtesy: Reuters photo)

US intelligence agencies have detected encrypted radio signals believed to be coming from Iran, triggering concerns that they could be linked to the activation of 'sleeper assets' overseas.

As per an alert reviewed by ABC News, 'preliminary signals analysis' of a transmission (likely originating from Iran) was detected across several countries shortly after the death of Ayatollah Ali Khamenei in a coordinated strike by US and Israel. The message may have been intended to trigger covert operatives using a coded signal.

The 'operational trigger', officials believe, closely matched the style of classic 'number stations'. These are mysterious shortwave radio broadcasts that typically consist of voices reading out seemingly random numbers. 'Number stations' date back to the Cold War era, when intelligence agencies like the CIA and KGB used them to secretly send encrypted instructions or messages to undercover spies and agents operating abroad. Such transmissions could only be decoded by someone with the matching one-time code.

"Tavajjoh! Tavajjoh!" — the Persian word for 'attention!' was the mysterious message that emerged on the airwaves shortly after the killing of Ayatollah Ali Khamenei on February 28. The male voice, according to a report in The LA Times, then read a seemingly random string of numbers.

The alert noted that the encrypted transmission may have been directed at 'covert operatives' or 'sleeper agents' active beyond the country of origin. Authorities have been urged to keep a close watch on any unusual radio-frequency signals.

Last week, President Donald Trump warned of Iranian sleeper cells operating in the United States. He said the US is monitoring threats against the country after he was briefed on the presence of these operatives on the American soil. "We know where most of them are. We got our eye on all of them, I think," he told reporters.

Amid the escalating US-Israeli military strikes on Iran, federal counterterrorism officials have also issued warnings about potential retaliation on American soil using sleeper cells. The LA Times reported that the retaliation could be through covert operatives embedded in US, affiliated terrorist groups, lone-wolf sympathizers or by launching cyberattacks against critical infrastructure.

"Sleeper cells have always been a concern when it comes to Iranians and their proxies. This isn't new, but given the situation, some of their proxies are feeling a lot more desperate," The LA Times said quoting Horace Frank, former head of counterterrorism for the Los Angeles police and a retired assistant chief.