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Russian hackers are believed to have breached the computers of the Clinton Foundation — and appear to be the same cyber spies who swiped confidential files from the Democratic Party and Hillary Clinton campaigns, officials familiar with the investigation told NBC News.

Three private security firms have concluded the hackers are Russian, and some security experts say Russian intelligence could be behind the attacks and want to interfere in the U.S. presidential election.

Related: 'Lone Hacker' Claims Responsibility for Cyber Attack on Democrats

So what could be the hackers' end game?

The Democratic National Committee said the operation was part of a Russian disinformation campaign. But some U.S. officials believe the goal is something more menacing: espionage.

A spokesman for the Bill, Hillary and Chelsea Clinton Foundation — the charity founded by the former president that supports humanitarian causes — said it has not been notified of any cyber attack.

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Bloomberg first reported Tuesday that the Clinton Foundation had been penetrated, citing three people familiar with the matter.

A hacker using the moniker “Guccifer 2.0” also released a new trove of documents Tuesday apparently taken from the DNC during a hack last week. Crowdstrike, the firm brought in by the Democrats to deal with the hack, was one of the firms attributing it to Russian intelligence agencies, which Russia denied.

“The attackers used advanced intrusion techniques to avoid detection and discovery. They were looking for information on policy, political campaigns and strategies, foreign policy plans,” Crowdstrike said in a statement.

"There has never before been a direct intervention in American politics by a foreign power that was this bold and went to these lengths."

This latest document dump, which comes after Gufficer 2.0 took responsibility for the DNC breach, included an internal assessment of issues over which the DNC thought Clinton was likely to be attacked on.

A Clinton campaign official told NBC News that it still has no evidence that its computers were specifically violated.

"However, what appears evident, is that Russian interests are trying to influence the outcome of the election," the official said.

Cyber experts 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.

Related: Hacker 'Guccifer': I Got Inside Hillary Clinton's Server

"There has never before been a direct intervention in American politics by a foreign power that was this bold and went to these lengths," said Scott Borg, director of the U.S. Cyber Consequences Unit, an independent nonprofit that assesses the impact of cyber attacks.

But three U.S. officials told NBC News that the U.S. intelligence community hasn’t determined that Russia is engaging in a wholesale attempt to interfere with the American election.

The cyber attack against the Clinton Foundation comes after Guccifer, a Romanian hacker who first exposed Clinton's private email address, said in May that he also gained access to the former Secretary of State's "completely unsecured" server. The hacker, Marcel Lehel Lazar, pleaded guilty in a U.S. court to related charges that same month.

It’s not clear whether those Guccifer-related leaks were orchestrated by Russian intelligence agencies as a way of specifically inflicting damage on the Clinton campaign, the U.S. officials said.