SAN FRANCISCO/WASHINGTON (Reuters) - The FBI is investigating a cyber attack against another U.S. Democratic Party group, which may be related to an earlier hack against the Democratic National Committee, four people familiar with the matter told Reuters.

Congressional candidates running for office and being supported by the Democratic Congressional Campaign Committee at the Democratic National Convention in Philadelphia. REUTERS/Rick Wilking

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 U.S. presidential election campaign to help Republican nominee Donald Trump.

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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 U.S. 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 was 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 U.S. officials have said there was evidence that Russia engineered the DNC hack to release sensitive party emails in order to influence the U.S. presidential election.

“I have concerns that an agency of foreign intelligence is hacking and interfering with a U.S. 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 U.S. Senator Bernie Sanders of Vermont, who ran a close race for the nomination for the Nov. 8 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.

Justin Harvey, chief security officer at Fidelis Cybersecurity company, said the suspect website in the hack was affiliated with others that host sophisticated malware undetected by the vast majority of antivirus providers.

“It’s really rare malware,” which would be more likely to be wielded by government hackers than ordinary criminals, he said.

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Russian officials have dismissed allegations of Moscow’s involvement in hacks of U.S. political groups. “It is so absurd it borders on total stupidity,” Kremlin spokesman Peskov said on Thursday.

Some Democratic officials have accused Russia of hacking the DNC emails in order to help Trump win the race for the White House.

“It’s no coincidence someone is hacking into Democratic Party computers. It’s almost sounding like a repeat of Watergate,” Gilberto Hinojosa, the chairman of the Texas Democrats said, after Reuters reported the DCCC hacking.

“This is just the kind of dirty politics we expect from Donald Trump. I have no doubt Donald Trump is behind it,” he said, citing the businessman’s praise of Russian President Vladimir Putin and recent remarks about Russia and Clinton’s deleted emails.

Trump angered Democrats this week by inviting Russia to unearth tens of thousands of emails from rival Clinton’s tenure as U.S. secretary of state. Trump said on Thursday his comment was meant to be sarcastic.

Jim Manley, a Democratic strategist who once worked for Senate Minority Leader Harry Reid, said the possibility of the DCCC being hacked was cause for great concern.

“Until proven otherwise, I would suggest that everyone involved with the campaign committee operate under the assumption Russians have access to everything in their computer systems,” Manley said.

The Federal Bureau of Investigation referred questions about the DCCC attack to a statement it made on Monday about the DNC hack:

“The FBI is investigating a cyber intrusion involving the DNC and are working to determine the nature and scope of the matter. A compromise of this nature is something we take very seriously, and the FBI will continue to investigate and hold accountable those who pose a threat in cyberspace.”

Director of National Intelligence James Clapper said on Thursday the U.S. intelligence community was not ready to “make the call on attribution” as to who was responsible for the DNC hack.

Clapper, speaking at the Aspen Security Forum, acknowledged: “There’s just a few usual suspects out there” who might be responsible for the cyber intrusion, suggesting it was the work of a state actor rather than an independent hacking group.

Clapper said in May he was aware of attempted hacks on campaigns and related groups and he expected to see more as the November election neared. The last two U.S. presidential cycles in 2008 and 2012 witnessed a barrage of cyber attacks from a range of adversaries targeting President Barack Obama’s campaign and the campaigns of his Republican foes, officials have said.