WASHINGTON (AP) — Chinese hackers targeted cellphones used by Republican presidential candidate Donald Trump and his running mate, JD Vance, two people familiar with the matter said Friday.

It was not immediately clear what, if any, data might have been taken, but U.S. officials are investigating, according to the people, who were not authorized to discuss an ongoing investigation publicly and spoke to The Associated Press on condition of anonymity.

An FBI statement did not confirm that Trump and Vance were among the potential targets, but said it was investigating “unauthorized access to commercial telecommunications infrastructure by actors affiliated with the People’s Republic of China.”

“Agencies across the U.S. government are working together to aggressively mitigate this threat and are coordinating with our industry partners to strengthen cyber defenses in the commercial communications sector,” the FBI said.

U.S. officials believe Trump and Vance are among several people whose phone numbers are believed to have been targeted and suspect it is part of a larger cyber espionage campaign launched by China, the person said.

The New York Times first reported that Trump and Pence were targeted and said the campaign was alerted by US officials this week.

Trump campaign spokesman Steven Cheung did not elaborate on the Chinese operation, but issued a statement accusing Democrat Kamala Harris’s campaign of emboldening foreign adversaries, including China and Iran.

The FBI has repeatedly warned over the past year about hacking operations in China, with FBI Director Chris Wray telling Congress in January that investigators disrupted a state-sponsored group known as Volt Typhoon. That operation targeted small office and home routers in the US owned by private citizens and companies. Their ultimate targets included the US’s water treatment plants, power grid, and transportation systems

last month, Wray said the FBI disrupted a separate campaign by the Chinese government which targeted universities, government agencies, and other organizations and installed malicious software on more than 200,000 consumer devices, including cameras, video players, and home and office routers.

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Michelle L. Price in New York contributed to this report.