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 disclosure is the latest sign that the Chinese burrowed deep inside U.S. communications networks in a far-reaching espionage campaign.

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