The FBI confirmed Monday for the first time that it is investigating the hack of the Democratic National Committee that resulted in the unauthorized release of at least 20,000 internal emails.

“The FBI is investigating a cyber intrusion involving the DNC” and is working “to determine the nature and scope of the matter,” the bureau said in a statement.

“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.”

A trove of internal DNC emails were posted online several days ago by WikiLeaks; law enforcement officials and cyber security experts have said Russian intelligence services appear to be behind the hack, an allegation echoed Monday by Hillary Clinton’s campaign manager, Robby Mook.

Mook told reporters that it appeared the Russians were seeking to help Republican nominee Donald Trump in the general election.

“The hack into the DNC was perpetrated by Russian state actors,” Mook said at a breakfast sponsored by Bloomberg Politics. “Considering the calculated release of these emails and the fact that it was Russian state actors that had the emails, experts have been inferring it was the Russians who perpetrated this leak for the purpose of helping Donald Trump and hurting Hillary Clinton.”