An ethics lawyer in the Obama administration said Tuesday that Donald Trump Jr.’s emails about setting up a meeting with a Russian lawyer who promised compromising information about Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE's campaign serves as "proof that collusion was offered and accepted."

"This is extraordinary: proof that collusion was offered, and accepted. whether actual help/info flowed in 1st meeting or later, irrelevant," Norm Eisen, who served in former President Obama's White House, wrote in a tweet.

this is extraordinary: proof that collusion was offered, and accepted. whether actual help/info flowed in 1st meeting or later, irrelevant https://t.co/pS5WVH7PRL — Norm Eisen (@NormEisen) July 11, 2017

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Eisen had shared a page from Trump Jr.'s email chain that he released Tuesday, which highlighted his correspondence with acquaintance Rob Goldstone about arranging a meeting with Russian lawyer Natalia Veselnitskaya.

Goldstone told Trump Jr. that Veselnitskaya had "information that would incriminate Hillary."

Trump Jr. said in one of his emailed responses: “If it's what you say I love it.”

Eisen said he has "never seen anything like this."

"I have worked on campaigns and as an election lawyer for decades; NEVER seen anything like this, [and] EVERY other campaign would have called FBI," Eisen wrote in another tweet.

The meeting took place in June 2016 at Trump Tower, two weeks after Trump Jr.'s father won the Republican presidential nomination. It included then-Trump campaign chairman Paul Manafort and Trump's son-in-law and now-White House adviser Jared Kushner.

While Trump Jr. denies that he received any compromising information about Clinton, the meeting with a lawyer, who has ties to the Kremlin, has raised eyebrows among both Democrats and Republicans.

The release of his email correspondence comes amid ongoing investigations, by a special counsel appointed by the Justice Department and several congressional panels, that are looking into whether Trump campaign aides colluded with Russia during the election.

Sen. Susan Collins Susan Margaret CollinsJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-Maine) on Monday called for the Senate Intelligence Committee to interview Trump Jr. about the meeting in question.

On Monday, the Kremlin denied knowledge of the meeting or that it knew Veselnitskaya.