A Chinese cyber-espionage group known as “Salt Typhoon” has infiltrated at least eight major U.S. telecommunications firms and stolen a large amount of cellphone data.

The cyber-attack, apparently ongoing for months, was part of a broader operation targeting telecommunications structures worldwide. While U.S. officials did not disclose any exact numbers, they confirmed that a vast number of Americans’ phone records had already been compromised, according to The Guardian.

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) announced in October a breach of “unauthorized access” to U.S. telecom networks by groups linked to China. However, as previously reported by The Dallas Express, similar attacks on Americans have seemingly been sprouting from China for years on end.

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After identifying the recently targeted cyber hacks, the FBI and CISA quickly tried to notify affected companies and their clients.

“We believe a large number of Americans’ metadata was taken,” the U.S. Official, choosing to remain anonymous, told reporters.“We do not believe it’s every cellphone in the country, but we believe it’s potentially a large number of individuals that the Chinese government was focused on.”

The recent attack from the Salt Typhoon group has affected a wide range of corporate giants, including industry giants such as Verizon, AT&T, and T-Mobile. The hacking campaign has also reportedly targeted not only customer data but also confidential internal communications information, with some of these attacks allegedly involving the theft of direct audio files from telephone calls.

U.S. intelligence officials, including Avril Haines, director of National Intelligence, and Jessica Rosenworcel, chairwoman of the Federal Communications Commission, held an emergency classified briefing earlier this month to explain the extent of the breaches and the steps being taken to mitigate future cyber attacks or data leaks in America.

As of the time of publication, T-Mobile has denied that any customer data was compromised, while Lumen Technologies, another telecom giant, has said that no evidence suggests that its networks were affected.

However, The Guardian reports that industry insiders still believe that the Salt Typhoon group’s operations are highly sophisticated and may have left very few traces in the wake of its recent cyberattacks.