Story highlights Recent cyber hacks have targeted state election boards and the DNC

Clinton issued an implicit threat to Russia that the US could hit back hard on cyber

(CNN) Republican presidential nominee Donald Trump disputed Monday that Russia may be behind a series of cyberattacks on election systems in the US and on the Democratic National Committee -- a conclusion that US officials and lawmakers have come to.

The New York real estate mogul questioned Russian culpability in response to comments by Democratic opponent Hillary Clinton at the first presidential debate pointing the finger at Russia for recent breaches.

"She says, 'Russia, Russia, Russia,' " Trump said. "It could be China. You don't know who broke into the DNC."

He continued, "It also could be somebody sitting on their bed that weighs 400 pounds."

But Clinton said there was "no doubt" that Russia has been responsible, an assessment top US security authorities and members of congressional intelligence committees support.

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