U.S. government officials said Tuesday that the China-backed hacking group dubbed Salt Typhoon are still inside some of the networks of America’s largest phone and internet providers, weeks after the long-running hacking campaign first came to light.

Cybersecurity agency CISA said in a call with reporters the affected telecom giants are still trying to evict the hackers, in part because it’s unclear what the hackers are aiming to accomplish.

News first broke in October that Salt Typhoon was reportedly deep inside the networks of AT&T, Verizon, and Lumen (formerly CenturyLink), among others. T-Mobile said it was targeted but largely rebuffed the attackers. The access allowed the Chinese hackers to access real-time unencrypted calls and text messages, as well as metadata about who the communications were sent to and from, as they traveled over the phone carriers’ networks.

U.S. officials believe the industry-wide hacks may be China trying to carry out a wide-ranging spying operation, as the hackers were found accessing the communications of U.S. officials and senior Americans, including presidential candidates. Salt Typhoon is also believed to be targeting systems that house much of the U.S. government’s requests, which may help to identify Chinese individuals under U.S. government surveillance.

On the call Tuesday, officials from CISA and the FBI urged Americans to use encrypted messaging apps to avoid having their communications intercepted by Chinese spies or other hacking groups.

“Encryption is your friend; whether it’s on text messaging or if you have the capacity to use encrypted voice communication,” said the CISA official. 

Signal and WhatsApp are among some of the most used end-to-end encrypted messaging apps, which prevent anyone — including the app makers — from accessing the communications of its users.The U.S. government also provided guidance for telecom networks on how to harden their networks from the China-backed hackers, noting that each victim company’s remediation efforts will be unique.