The FBI has launched an investigation into the hack of the Democratic National Committee (DNC), the law enforcement agency said on Monday.

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"The FBI is investigating a cyber intrusion involving the DNC and is working to determine the nature and scope of the matter," it 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.”

On Friday, WikiLeaks published 20,000 emails stolen from the committee, many of them showing an effort by DNC officials to boost Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE's presidential primary bid.

DNC Chairwoman Debbie Wasserman Schultz announced on Sunday that she would step down following the four-day Democratic National Convention this week in Philadelphia.

Security experts say the hack, first reported in June, was the work of Russian intelligence groups. The DNC itself has pinned the theft on Russia.

Initially, the leak was limited to the committee's opposition research on Republican nominee Donald Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE.

But the timing of Friday's dump of emails — just days before the convention to formally name Clinton as the party's standard-bearer — has sparked fears that Russia is attempting to manipulate the U.S. election to benefit the Republican nominee.

Clinton’s campaign manager, Robby Mook, argued on ABC’s “This Week” on Sunday that the emails were “leaked by the Russians for the purpose of helping Donald Trump.”

"I don't think it's coincidental that these emails were released on the eve of our convention here, and I think that's disturbing," Mook said Sunday on CNN's "State of the Union."

Trump has pushed back on the theory.

“The new joke in town is that Russia leaked the disastrous DNC e-mails, which should never have been written (stupid), because Putin likes me," he tweeted early Monday.

The contents of the emails have plunged the convention into chaos, and by Monday morning, some polls already showed that Trump has regained the lead in the presidential race for the first time since May.

The FBI's investigation into the politically charged hack comes just weeks after it closed its contentious probe of the private email server Clinton used when she was secretary of State. FBI Director James Comey and the Department of Justice faced fierce criticism for his decision not to recommend criminal charges against Clinton.

Updated at 12:38 p.m.