The FBI is investigating a cyber intrusion at the Democratic Congressional Campaign Committee (DCCC) that may be related to an earlier hack at the Democratic National Committee, said four sources familiar with the matter on Thursday.

The previously unreported incident at the DCCC, which raises money for Democrats running for seats in the US House of Representatives, may have been intended to gather information about donors, rather than to steal money, the sources said.

The breach and its potential ties to Russian hackers are likely to sharpen concern, so far unproven, that Moscow is attempting to meddle in US elections. The issue has clouded this week’s Democratic national convention in Philadelphia.

The DCCC intrusion could have begun as recently as June, two of the sources told Reuters.

That was when a spoof 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 spoof site, two sources said.

The sources said the internet protocol address of the spurious site resembled one used by a Russian government-linked hacking group, one of two such groups suspected in the breach of the DNC, the nationwide strategy setting and money-raising body for the Democratic party.

The DCCC had no immediate comment. Donation processing company ActBlue had no immediate comment.

The FBI referred questions to a statement it made on Monday on 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.”

While private cyber experts and the government were aware of the DNC hack months ago, embarrassing emails were leaked last weekend by the WikiLeaks anti-secrecy group just as the party prepared to anoint Hillary Clinton as its presidential candidate for the 8 November election.

The revelation of the DCCC breach is likely to further stoke concerns among Democratic party operatives, many of whom have acknowledged they fear further dumps of hacked files that could harm their candidates. WikiLeaks has said it has more material related to the US election that it intends to release.

The DNC chair, Debbie Wasserman Schultz, resigned after the leaked emails this past weekend showed party leaders favoring Clinton over her rival in the nomination campaign, Senator Bernie Sanders. The committee is supposed to be neutral.

Cybersecurity experts and current US officials, who asked not to be identified, have said there is strong evidence that Russia was responsible for the DNC breach.

The director of national intelligence, James Clapper, said on Thursday the US intelligence community was not ready to “make the call on attribution” as to who was responsible for the DNC hack. The White House said earlier the FBI had not disclosed any information about who was behind the hack.

Clapper, speaking at the Aspen Security Forum, acknowledged that “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.

Russian officials have dismissed the allegations of Moscow’s involvement. “It is so absurd it borders on total stupidity,” said Kremlin spokesman Dmitry Peskov.