By Evan Perez

WASHINGTON — (CNN) The FBI on Monday confirmed they are investigating a hack into the Democratic National Committee, the first acknowledgment from the agency that they are probing the incident. Fallout over the emails led DNC Chairwoman Debbie Wasserman Schultz to announce her resignation Sunday.

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

Over the weekend, Wikileaks began publishing emails from the DNC. The group didn’t identify the source. But the campaign of presumptive Democratic nominee Hillary Clinton pointed the finger at Russia, saying the release of stolen emails was intended to help Republican nominee Donald Trump.

The FBI is investigating the DNC hack and has sent experts to meet with the Republican National Committee, as well as the major campaigns, to discuss their security measures, the officials say. No similar intrusions have so far been detected at the RNC or the campaigns of the two major party candidates, the officials say.

Clinton campaign manager Robby Mook told CNN’s Jake Tapper on “State of the Union” Sunday about that “changes to the Republican platform to make it more pro-Russian,” which could provide some of the motive behind the hacks.

“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,” he said.

Trump told The New York Times in an interview last week, that if he’s elected the US wouldn’t defend NATO allies against Russian aggression if they haven’t “fulfilled their obligation to us.”

Trump’s son, Donald Trump Jr., denied that his father’s campaign had anything to do with encouraging Russians to hack the DNC.

“I can’t think of bigger lies, but that exactly goes to show you what the DNC and what the Clinton camp will do,” Trump told Tapper on “State of the Union”.

James Trainor, assistant director for FBI’s cyber division, told CNN in a recent interview that the bureau has been working with political organizations and think tanks to put more resources into the security of their computer networks. He wouldn’t discuss the DNC or the role of Russia, but spoke generally about the increased number of such intrusions.

“There’s been aggressive targeting of that sector, the various campaigns, think tanks in the Washington, DC area,” Trainor said.

The “targeting of any candidate or any party (that) has political intelligence,” Trainor said. “There’s value in information there if you’re a nation state actor, so (it) shouldn’t be surprising.”

Private-sector cyber security investigators hired by the DNC concluded that hackers working for the Russian government were behind the year-long breach of the DNC. The investigation found intrusions by two Russian hacking groups.

At least one of the DNC intruders is believed to be behind other breaches of non-classified US government systems at the White House, State Department and other agencies, security experts believe, based on an analysis of malicious software in the breaches.

The FBI and other US agencies involved haven’t yet officially attributed the DNC attack to Russian government hackers. But evidence gathered by the FBI so far points to groups that are known to US counter-intelligence for carrying out intrusions for the Russian government, the officials briefed on the probe say.

The release of the emails over the weekend, however, raised new questions among government and private sector security officials.

It’s possible that other hackers took advantage of the DNC’s vulnerabilities and also stole information, US officials said. But the intrusion so far appears to bear the hallmarks of a Russian intelligence operation.

Foreign spy agencies routinely try to collect information on US elections, and there were some cyber attacks against political campaigns detected during the 2012 election cycle.

Typically, spy agencies collect such information to try to better inform their governments about US politics. US spy agencies do the same overseas.

Russian spy agencies have published embarrassing information to try to influence political events in countries they consider part of their sphere of influence. But to publicly release vast troves of stolen data to try to influence a US election is beyond the scale of what US counterintelligence officials have seen.

Whether Russian intelligence agencies provided the stolen information to Wikileaks, either directly or through middlemen, is now a top issue for US investigators to resolve, the US officials briefed on the probe say. The answer won’t likely come until well after the election.

The impact of the release of the DNC hacked emails was almost immediate, prompting the ouster of the head of one of the two major political parties.

Since the hackers were in the DNC systems for about a year, US officials expect more data releases.

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