The computers of the Democratic campaign committee in the US Congress have been hacked in an intrusion that resembles the recent cyber breach of the Democratic National Committee, a spokeswoman said.
The incident added another layer of mystery to the hacking of Democratic Party information that has been revealed in the heat of this year's presidential and congressional elections.
Details were initially unclear about who tapped into the computers and which information was accessed at the Democratic Congressional Campaign Committee, with spokeswoman Meredith Kelly saying the committee was "the target of a cybersecurity incident".
The organisation raises money and provides other assistance for the party's House candidates.
President Barack Obama has said Russia was almost certainly responsible for the hack of the Democratic National Committee, an assertion with which cybersecurity experts have agreed.
That breach led to the release by WikiLeaks on July 22, days before the Democratic National Convention, of 19,000 emails showing that supposedly neutral party officials were favouring Hillary Clinton over Bernie Sanders during their primary contest for the presidential nomination.
As a result of that disclosure, party chairwoman Debbie Wasserman Schultz announced her resignation this week.
Ms Kelly said the campaign committee is using CrowdStrike, a computer security firm based in California, and is "co-operating with the federal law enforcement with respect to their ongoing investigation". She said her organisation is "continuing to take steps to enhance the security of our network in the face of these recent events".
A House Democratic aide said the FBI is investigating the hack.
CrowdStrike issued a statement confirming its work for the congressional campaign committee but provided no additional details.
Computer hacking, emails and indications of Russian involvement have evolved into a political issue in the presidential campaign between Ms Clinton and Republican candidate Donald Trump.
This week, Mr Trump encouraged Moscow to seek and release more than 30,000 other missing emails deleted by Ms Clinton, the former secretary of state. Democrats accused him of trying to get a foreign adversary to conduct espionage that could affect November's election, but Mr Trump said he was merely being sarcastic.