If Russia is behind the hacking of servers at the Democratic National Committee, the U.S. should prove it, former NSA contractor Edward Snowden said Monday.

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The whistleblower, who fled to Russia in 2013 to avoid extradition after he leaked secrets about U.S. surveillance programs, said policies set forth by the U.S. director of national intelligence often prevent the information from being shared. The biggest reason has been to protect knowledge of the existence of U.S. surveillance methods, especially a program Snowden exposed known as "XKeyscore" that allows intelligence operatives to surveil global Internet data.

Snowden said the policy was antiquated and only served to encourage state-sponsored hacking. "If Russia hacked the #DNC, they should be condemned for it. But during the #Sony hack, the FBI presented evidence," Snowden said on Twitter, referring to the 2014 breach of Sony Pictures Entertainment backed by North Korea.

"Even if the attackers try to obfuscate origin, #XKEYSCORE makes following exfiltrated data easy. I did this personally against Chinese ops," Snowden said. "Evidence that could publicly attribute responsibility for the DNC hack certainly exists at #NSA, but DNI traditionally objects to sharing … The aversion to sharing #NSA evidence is fear of revealing 'sources and methods' of intel collection, but #XKEYSCORE is now publicly known.

"Without a credible threat that [the U.S. government] can and will use #NSA capabilities to publicly attribute responsibility, such hacks will become common," he added. "This is the only case in which mass surveillance has actually proven effective. Though I oppose in principle, it is a mistake to ignore.

"To summarize: the US Intel Community should modernize their position on disclosure. Defensive capabilities should be aggressively public," Snowden said.

A breach of DNC servers announced in June led to the exposure of approximately 20,000 emails sent or received by Democratic Party officials. At least two of the infiltrators were traced to groups affiliated with the Russian government, though experts have suggested many more were likely present on the system.

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The statement by Snowden, who remains protected in Russia, is notable, though it also serves as a critique of President Obama's cybersecurity policies. The president has strenuously sought to avoid attributing responsibility for cyberattacks against the U.S., to the chagrin of members of Congress and often even national security officials.

Snowden, who faces charges related to exposing classified information, has criticized both Hillary Clinton and Donald Trump. He has been relatively sympathetic to just one candidate, Vermont Sen. Bernie Sanders.