The hacker who goes by the name "Guccifer 2.0" on Friday released files purportedly stolen in a cyberattack on the Democratic Congressional Campaign Committee.
The documents posted online do not appear to contain any email or communications, but rather include shared passwords for the committee shared accounts to various news services, Lexis, and a federal courts public access system called PACER.
Documents purportedly showing Congressional contact lists and campaign overviews were also released.
A committee spokesperson said they are cooperating with federal authorities investigating the cyberattack. "We are aware of reports that documents claimed to be from our network have been released and are investigating their authenticity," DCCC national press secretary Meredith Kelly said in a statement.
The intrusion in the Congressional committee's computer system was disclosed in late July. At the time, officials said it appears similar to a hack on the Democratic National Committee.
The Democratic National Convention was thrown into some early controversy when internal emails between DNC officials were posted by the website WikiLeaks days before the convention began. Those emails appeared to show a bias among some officials in favor of Hillary Clinton, which angered some Bernie Sanders supporters.
The cyberattacks have raised concerns among U.S. national security officials that outside groups or governments could be trying to interfere in the presidential election.
Many U.S. security officials and cyber security experts have concluded that state-sponsored Russian hackers stole the DNC emails, NBC News reported last month.
The DCCC said a cyber security company it hired after the breach concluded two Russian security agencies were behind the that hack.
Cyber experts have told NBC News they believe Guccifer 2.0 is a Russian front, and also said the leaks show that Russia is seeking to influence the U.S. presidential campaign, perhaps with an eye toward helping Donald Trump.
Russia has denied being behind the cyberattacks.
Rep. Adam Schiff, a Democrat representing California who is ranking member of the House Permanent Select Committee on Intelligence, said in a statement Friday that "The unauthorized disclosure of people's personally identifiable information is never acceptable."
"I have every confidence that law enforcement will get to the bottom of this, and identify the responsible parties," Schiff said. "And when they do, I hope the Administration will disclose who is attempting to interfere with the American political process, and levy strong consequences against those responsible."