Sen. Richard Blumenthal is asking Senate Judiciary Chairman Chuck Grassley, R-Iowa, to “immediately” subpoena Donald Trump Jr. for public testimony and documents.

Trump Jr., the eldest child of President Trump, was grilled by the House Intelligence Committee privately on Wednesday but has yet to testify in a public setting before any of the congressional committees investigating Russian meddling in the 2016 presidential election and possible collusion with the Trump campaign.

In a letter to Grassley, the Connecticut Democrat said only after a subpoena followed by public testimony and documents will the “American people […] feel certain that Mr. Trump, Jr. has been fully forthcoming.”

Blumenthal cites “stunning” reports recently — including Twitter messages Trump Jr. exchanged with WikiLeaks during the 2016 campaign — that suggest collusion between Russia and the Trump campaign.

Trump told the Senate Judiciary Committee in a private interview in September he corresponded with the groups during the campaign, but only revealed the messages after a story about them by the Atlantic last month.

“Though he claimed to be fully cooperating with the Judiciary Committee investigation, Mr. Trump, Jr. did not initially turn over these communications. […] Only after he was questioned under oath did he reveal the existence,” of them, Blumenthal writes to Grassley, and the committee has been unable to ask him about the messages since.

Blumenthal says Trump Jr.’s “failure to be forthcoming is part of a pattern,” citing three different “incomplete and misleading” statements given by the president’s son and his lawyer about his now infamous June 9, 2016, meeting with Russians at Trump Tower.

Trump Jr.’s “refusal” to testify before Senate Judiciary publicly means that “key questions have been left unanswered.”

Questions related to Russia and the Trump campaign “can only be answered if Mr. Trump, Jr. is compelled to fully cooperate with the committee’s investigation,” Blumenthal writes, asking Grassley issue a subpoena on behalf of the committee immediately.

Only then will he have to “fully account for his actions in front of the American people and to provide this committee with the documents we need to fulfill our important oversight requests,” Blumenthal concludes.