The Department of Justice and the FBI declined to comment on whether candidates were targeted.
A joint statement, external from the FBI and the Infrastructure Security Agency (CISA) said the US government was investigating the “unauthorised access to commercial telecommunications infrastructure by actors affiliated with the People’s Republic of China”.
They said after the “malicious activity” was identified, the agencies “immediately notified affected companies, rendered technical assistance, and rapidly shared information to assist other potential victims,” adding that the investigation was ongoing.
“Agencies across the US government are collaborating to aggressively mitigate this threat and are coordinating with our industry partners to strengthen cyber defences across the commercial communications sector,” they added.
The Trump campaign blamed Democrats for the hack, claiming without evidence that it was an attempt “to prevent President Trump from returning to the White House”.
Law enforcement is currently treating the hack as an act of espionage, not as an attempt at campaign influence, one source told CBS.
Earlier this month it emerged that US telecommunications companies had been targeted in a hack.
One of the companies affected is said to be Verizon, through which the hackers are thought to have potentially targeted Trump and Vance’s data, according to the New York Times, who first reported the story.
In a statement, Verizon spokesman Rich Young said the company was “aware that a highly sophisticated nation-state actor has reportedly targeted several US telecommunications providers to gather intelligence.”
He said Verizon is assisting law enforcement agencies in the investigation and working to address any further problems.
The Trump campaign has already been the target of one hack earlier this year.
Three Iranians nationals linked to the country’s Islamic Revolutionary Guard Corps were charged in September with deliberately attempting to undermine a presidential campaign.
US government agencies and officials have long-warned of the threat of foreign interference in the US, including US elections.
“Our adversaries do look at American elections as points to try to influence, to try to undermine confidence in our democracy, to try to put their thumb on the scale,” National Security Adviser Jake Sullivan said in the summer. “We are clear eyed about that. And we are doing a lot to push back against it”.
In January, the issue was discussed in Congress, with FBI Director Christopher Wray warning that Chinese hackers were preparing to “wreak havoc and cause real-world harm” to the US.