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.

The FBI and the Cybersecurity and Infrastructure Security Agency (CISA) said that People's Republic of China (PRC) hackers breached commercial telecommunication service providers in the U.S. The breached entities have been warned, and the agencies are ...