The Federal Bureau of Investigation is investigating a cyber attack against another US Democratic Party group, which may be related to an earlier hack against the Democratic National Committee, four people familiar with the matter told Reuters.
The previously unreported incident at the Democratic Congressional Campaign Committee, or DCCC, and its potential ties to Russian hackers are likely to heighten accusations, so far unproven, that Moscow is trying to meddle in the US presidential election campaign to help Republican nominee Donald Trump.
The Kremlin denied involvement in the DCCC cyber-attack. Hacking of the party's emails caused discord among Democrats at the party's convention in Philadelphia to nominate Hillary Clinton as its presidential candidate.
The newly disclosed breach at the DCCC may have been intended to gather information about donors, rather than to steal money, the sources said on Thursday.
It was not clear what data was exposed, although donors typically submit a variety of personal information including names, email addresses and credit card details when making a contribution. It was also unclear if stolen information was used to hack into other systems.
The DCCC raises money for Democrats running for seats in the US House of Representatives. The intrusion at the group could have begun as recently as June, two of the sources told Reuters.
That was when a bogus website was registered with a name closely resembling that of a main donation site connected to the DCCC.
For some time, internet traffic associated with donations that were supposed to go to a company that processes campaign donations instead went to the bogus site, two sources said.
The sources said the Internet Protocol address of the spurious site resembled one used by Russian government-linked hackers suspected in the breach of the DNC, the body that sets strategy and raises money for the Democratic Party nationwide.
Cyber security experts and US officials have said there was evidence that Russia engineered the DNC hack to release sensitive party emails in order to influence the US presidential election.
"I have concerns that an agency of foreign intelligence is hacking and interfering with a US election," said Clinton campaign chairman John Podesta, who added he had not seen news of the DCCC attack.
Asked by Reuters to comment on whether Russia played a role in a cyber-attack on the DCCC, Kremlin spokesman Dmitry Peskov said, ''We don't see the point any more in repeating yet again that this is silliness.''
The release of the DNC emails by activist group WikiLeaks caused uproar in the party because they appeared to show favoritism within the DNC for Clinton over US Senator Bernie Sanders of Vermont, who ran a close race for the nomination for the 8 November election. The committee is supposed to be neutral.
The DNC and the DCCC share the same office space on South Capitol Street in Washington.
The DCCC and donation processing company ActBlue had no comment on Thursday. CrowdStrike, the California-based cyber security firm that investigated the DNC breach, declined to comment.