U.S. authorities are confident Russian intelligence agencies are behind the hack of Democratic Party emails, and those in the Clinton campaign are freaking out. Political operatives and cybersecurity experts have revealed that Americans should brace 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: how destructive the October Surprise will be to Democratic nominee Hillary Clinton and to broader U.S. foreign policy efforts.

Democratic Party and Clinton campaign officials are now frantically conducting “damage assessments” 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.”

Julian Assange, whose WikiLeaks organization leaked more than 20,000 DNC emails last Friday, has promised to leak more in an effort to damage Clinton.

The cybersecurity firm hired by the DNC 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.