Chinese-linked hackers accessed surveillance data meant for U.S. law enforcement after infiltrating the networks of several telecom companies, according to a statement from U.S. authorities on Wednesday.

The U.S. government has confirmed that hackers with links to China breached multiple U.S. telecommunication service providers to access the wiretap systems used by law enforcement to surveil Americans.

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