Story highlights Donald Trump Jr. was emailed about meeting with a Russian lawyer during last year's campaign

The oldest son of the President released a screenshot Tuesday apparently of those messages

(CNN) Hillary Clinton's former running mate, Sen. Tim Kaine, said Donald Trump Jr. may have committed treason when he agreed to meet with a Russian lawyer under the pretense of getting dirt on Clinton.

"That, the investigation -- it, it's not -- nothing is proven yet. But, we're now beyond obstruction of justice in terms of what's being investigated," the Virginia Democrat told CNN Tuesday. "This is moving into perjury, false statements and even potentially treason."

Kaine's comments follow a series of New York Times reports including one story Monday on an email sent to Trump Jr. last year stating that a Russian lawyer had "compromising" information about Hillary Clinton as "part of a Russian government effort" to help the Trump campaign. President Donald Trump's oldest son tweeted Tuesday screenshots of an email exchange that he says are the emails setting up the meeting with the lawyer. That tweet came after Kaine's comment.

A Kaine spokesperson later sent a statement to CNN saying that the matter of treason will be up to investigators to prove.

"Senator Kaine stressed that nothing is proven yet," the Kaine spokesperson said. "But he acknowledged the grave impropriety, if proven, of any effort to cooperate with a foreign nation -- especially one deemed by our military leaders to be a primary state adversary -- to influence an American election."

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