Encrypted communications, believed to originate from Iran and possibly serving as an “operational trigger” to activate “sleeper cells” in foreign countries, have been detected by the United States, according to a federal notice to law enforcement reviewed by ABC News.

The notice cites a “preliminary signals analysis” regarding a transmission of “possible Iranian origin” that was relayed through multiple countries shortly after the death of Ayatollah Ali Khamenei following a U.S.–Israeli attack in Tehran on February 28.

The transmission was encoded and appears to have been intended for “covert recipients” who possess the corresponding decryption key. This type of message is used to transmit instructions to “covert agents or sleeper cells” without using the internet or mobile phone networks.

Such transmissions, the notice states, “are intended to activate or provide instructions to pre-positioned sleeper cells operating outside the country of origin.”

“Although the exact content of the transmissions cannot currently be determined, the sudden appearance of a new station with characteristics of international relay justifies increased operational vigilance,” it adds.

The same notice emphasizes that “there is no operational threat linked to a specific location,” but calls on law enforcement agencies to increase monitoring of suspicious activity on radio frequencies.