The FBI and CISA previously said they were investigating breaches by a China-backed hacking group inside several telecommunications providers, but had not said whether any data was stolen or whether the hackers accessed the systems used to fulfill legal wiretap requests.

An FBI investigation into reported hacks at AT&T and Verizon reveals 'a broad and significant cyber espionage campaign,' the agency says.

China-affiliated actors "have compromised networks at multiple telecommunications companies" to enable the theft of U.S. customer call records and access to the private communications of some individuals involved in government or politics,