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.
"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.