Surveillance systems that track the locations of cellphone users and spy on their calls, texts and data streams are being turned against Americans as they roam the country and the world, say security experts and U.S. officials.

Federal officials acknowledged the privacy risk to Americans in a previously undisclosed letter from the Department of Homeland Security to Sen. Ron Wyden, D-Ore., last week, saying they had received reports that “nefarious actors may have exploited” global cellular networks “to target the communications of American citizens.”

The letter, dated May 22 and obtained by The Washington Post, described surveillance systems that tap into a global messaging system that allows cellular customers to move from network to network as they travel. The decades-old messaging system, called SS7, has little security, allowing intelligence agencies and some criminal gangs to spy on unwitting targets — based on nothing more than their cellphone numbers.

“I don’t think most Americans realize how insecure U.S. telephone networks are,” Wyden said in a statement. “If more consumers knew how easy it is for bad guys to track or hack their mobile phones, they would demand the FCC and wireless companies do something about it. These aren’t just hypotheticals.”