Now that U.S. authorities are confident Russian intelligence agencies are behind the hack of Democratic Party emails, political operatives and cybersecurity experts tell NBC News they are bracing for an “October Surprise” -– a release of even more potentially damaging information timed to influence the outcome of the presidential election and the course of the next administration.

The big question isn’t whether more information will be disclosed, they say, but how destructive it might be to Democratic nominee Hillary Clinton and to broader U.S. foreign policy efforts.

Democratic Party and Clinton campaign officials are now doing an urgent “damage assessment” to determine what kind of information might have been stolen and the impact its release might have on a tight presidential race.

“That is a nightmare scenario, and let’s hope we don’t see that as an October Surprise –- emails from Hillary Clinton’s server that have either been in the press or worse, the classified ones that no one in the public has seen,” said retired Adm. James Stavridis, who as the former Supreme Allied Commander for NATO is familiar with Russian information operations.

“I think it is a nightmare for all of us because it shows the degree to which our systems have been penetrated by Russian hackers potentially operating under the rubric of the Russian government,” said Stavridis, who was vetted as a possible Clinton running mate.

Julian Assange, whose WikiLeaks organization leaked more than 19,000 DNC emails last Friday, has promised to leak more in an effort to damage Clinton.

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The cybersecurity firm hired by the Democratic National Committee to investigate the hacks says that two Russian security agencies had been accessing DNC servers and internal files for months, with at least one of them infiltrating the Joint Chiefs of Staff and other key U.S. agencies.

Russian hackers also accessed the private email accounts of some Clinton campaign staffers, and attacked and may have accessed internal files and email servers of the Clinton Foundation. Security officials also believe hackers accessed the private server Clinton used while Secretary of State.

The U.S. intelligence community has been gravely concerned for more than a decade about ramped-up cyber-penetrations by Russia, said Fiona Hill, who from 2006 to 2009 served as the Russia/Eurasia intelligence officer on the National Intelligence Council.

As a former lieutenant colonel in Russia’s spy agency, Putin considers himself a spook, Hill said, and as such is the world’s most aggressive user of such intelligence to meddle in his adversaries’ affairs and gain an upper hand in geopolitical power struggles.

“It’s much more nuanced than just influencing the election; it’s more about putting a spotlight on the United States’ failings and damaging the credibility of our political system and our political elites,” said Hill, co-author of the 2015 book, “Mr. Putin: Operative in the Kremlin.”

“The advantage of that [for Putin] is that it will be extraordinarily difficult for whoever comes into office in January to forge a coherent foreign policy. The United States will be pretty wounded on the global stage.”

Like others, Hill also speculated that if the trail does lead back to Putin, it might have something to do with his belief that Clinton, as secretary of state, tried to undermine his successful 2011 campaign to regain the Russian presidency.

“So why shouldn’t he have a go at that -- of having a subversive effect on our politics?” Hill asked. “It’s conceivable that he’s serving it right back at her, and undermining her election effort.”

Mark Galeotti, a longtime Putin observer and senior research fellow at the Institute of International Relations in Prague, said Moscow clearly sees Clinton as an enemy of the state.

“However, I don't think they necessarily want or expect to see a Trump presidency,” he said. “Rather, they want a weak Clinton White House that is too mired in domestic disputes and struggling to achieve a mandate at home to be that effective or aggressive abroad.”

Others who are familiar with Putin say they fear the worst: a full-scale assault on the American electoral process.

“They could do a lot of things with the information they‘ve gathered, now and after the convention,” said Evelyn Farkas, who was the Pentagon’s top official overseeing military relations with Russia until last September.

Farkas says she’s afraid the Russians will somehow find a way to disrupt or garble vote counting and other aspects of an increasingly computerized campaign and election process. “What I really worry about is the security of our electronic electoral system.”