The head of the FBI on Tuesday refused to rule out the possibility that hackers may have broken into Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE’s private email server.

Federal investigators found no direct evidence that hackers had managed to break into the server or access the email account hosted on it, Director James Comey told reporters on Tuesday, while announcing that the FBI would not recommend charges against the former secretary of State and presumptive Democratic presidential nominee.

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“But, given the nature of the system and of the actors potentially involved, we assess that we would be unlikely to see such direct evidence,” he said.

“Hostile actors” did gain access to people with whom Clinton was regularly communicating, he noted, such as longtime associate Sidney Blumenthal. The existence of her private email account was also relatively well-known, and she used it while in foreign countries that possess “sophisticated” cyber operations.

So Comey could not rule out that a hacker somewhere had managed to break into her system, unbeknownst to the federal government.

“Given that combination of factors, we assess it is possible that hostile actors gained access to Secretary Clinton’s personal email account,” Comey said.

Notorious Ukrainian hacker “Guccifer” boasted earlier this year about having broken into Clinton’s server, though there was little evidence to back up the claim.

Hackers tied to Russia have breached the Clinton campaign, in what intelligence officials have warned is direct targeting of U.S. presidential candidates by foreign intelligence services

Possible security vulnerabilities on Clinton’s server have been a target of derision by her political opponents, who warn that the arrangement may have handed reams of sensitive government secrets over to foreign agents and domestic hackers.

Little is known about Clinton’s setup, but Comey on Tuesday acknowledged that it was “not even supported by full-time security staff,” unlike government email systems or those of commercial outfits such as Google's Gmail.