Chinese state-affiliated hackers have collected audio from the phone calls of U.S. political figures, according to three people familiar with the matter. Those whose calls have been intercepted include an unnamed Trump campaign adviser, said one of the people.

The hackers are said to be part of a Chinese government-affiliated group that American researchers have dubbed Salt Typhoon and were able to collect audio on a number of calls as part of a wide-ranging espionage operation that began months ago, according to the people, who spoke on the condition of anonymity because a federal investigation is underway. The government is still seeking to determine how much audio the hackers have, one of the people said.

They were also able to access unencrypted communications, including text messages, of the individual, the people said. End-to-end encrypted communications such as those on the Signal platform are believed to have not been hacked, they said.

The development heightens concerns over the extent of the infiltration as the 2024 election is in high gear as well as the potential threat to long-term national security.

The FBI declined to comment on the matter.

The FBI and other U.S. agencies are still investigating the full extent and nature of the espionage campaign. The hackers targeted the phones of former president Donald Trump, who is running to regain the White House, and his running mate JD Vance, the New York Times first reported Friday. They were thought to have targeted information about call logs, and there is no evidence so far that the hackers listened in on calls of the two Republicans at the top of the ticket.

As previously reported, Democrats were also targeted in the hacking efforts, including the staff of Sen. Majority Leader Charles E. Schumer (D-New York), according to one of the people familiar with the matter.

The Salt Typhoon group is also thought to have targeted the system that tracks lawful requests for wiretaps made by the federal government of carriers. The motive there could be to figure out who the FBI and other federal agencies have under surveillance, said people familiar with the matter.

The matter is so serious that the White House earlier this month set up an emergency multiagency team to ensure all relevant agencies have visibility into the investigation. The establishment of a “unified coordination group” triggers a separate mandatory investigation by a public-private Cyber Safety Review Board, which in this case will probe the lapses that led to the intrusions. The board is led by the Department of Homeland Security and includes cyber experts from industry. It’s unclear when the probe will begin, officials said.

The wide-ranging operation has involved at least 10 telecom companies, including major carriers such as AT&T, Verizon and Lumen.

At least one U.S. official was notified late last week that a personal cellphone had been accessed by the Salt Typhoon hackers, said one of the people familiar with the matter. The hackers were targeting phone logs, SMS text messages and other data on the device, said the person. It was not clear whether audio calls were successfully intercepted for that official, the person said.

Aaron Schaffer contributed to this report.