The latest attack follows two other attacks by hackers, whom U.S. intelligence officials have concluded were Russian.

A computer network used by Democratic presidential nominee Hillary Clinton’s campaign was hacked as part of a broad cyber attack on Democratic political organisations, people familiar with the matter told Reuters.

The latest attack, which was disclosed to Reuters on Friday, follows two other hacks on the Democratic National Committee, or DNC, and the party’s fundraising committee for candidates for the U.S. House of Representatives.

A Clinton campaign spokesman said in a statement late on Friday that an analytics data programme maintained by the DNC and used by the campaign and a number of other entities “was accessed as part of the DNC hack”.

“Our campaign computer system has been under review by outside cyber security experts. To date, they have found no evidence that our internal systems have been compromised,” said Clinton campaign spokesman Nick Merrill.

Later, a campaign official said hackers had access to the analytics programme’s server for approximately five days. The analytics data programme is one of many systems the campaign accesses to conduct voter analysis, and does not include social security numbers or credit card numbers, the official said.

The U.S. Department of Justice national security division is investigating whether cyber attacks on Democratic political organisations threatened U.S. security, sources familiar with the matter said on Friday.

The involvement of the Justice Department’s national security division is a sign that the Obama administration has concluded that the hacking was sponsored by a state, people with knowledge of the investigation said.

While it is unclear exactly what material the hackers may have gained access to, the third such attack on sensitive Democratic targets disclosed in the last six weeks has caused alarm in the party and beyond, just over three months before the November 8, 2016 U.S. presidential election.

Hackers, whom U.S. intelligence officials have concluded were Russian, gained access to the entire network of the fundraising Democratic Congressional Campaign Committee, or DCCC, said people familiar with the matter, detailing the extent of the breach to Reuters for the first time.

Cyber security experts and U.S. officials said earlier this week they had concluded, based on analysis of malware and other aspects of the DNC hack, that Russia engineered the release of hacked Democratic Party emails to influence the U.S. presidential election.

The U.S. Federal Bureau of Investigation said on Friday it was “aware of media reporting on cyber intrusions involving multiple political entities, and is working to determine the accuracy, nature and scope of these matters”.

“The FBI takes seriously any allegations of intrusions, and we will continue to hold accountable those who pose a threat in cyberspace,” the agency said in an emailed statement.

The hack did not involve the private email system Ms. Clinton used while she was Secretary of State.