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.

The U.S. government said it is investigating after People's Republic of China hackers targeted commercial telecommunications service providers in the U.S.

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