Sen. Tim Kaine (D-Va.) said the investigation into Russia’s interference in last year’s presidential election is moving beyond simply obstruction of justice and “potentially” into grounds for treason.

“The investigation ― it’s not, nothing is proven yet, but we’re now beyond obstruction of justice in terms of what’s being investigated,” the 2016 Democratic vice presidential candidate told reporters on Tuesday.

“This is moving into perjury, false statements and even into potentially treason,” he said.

Kaine’s comments come in the wake of reports that Donald Trump Jr., President Donald Trump’s son, met with a Russian lawyer in June 2016 in hopes of gaining damning information about Hillary Clinton, the elder Trump’s Democratic rival in the White House race. On Monday, The New York Times reported that Trump Jr. was informed in advance that the potentially damaging information about Clinton had been sourced to the Kremlin.

Clinton, who has been active on Twitter, has not yet weighed in on the matter. A spokesman for her did not immediately return a request for comment. Kaine was the vice presidential running candidate on Clinton’s ticket.

Senate Minority Leader Chuck Schumer (D-N.Y.) on Monday called on Trump Jr. to testify before the Senate Intelligence Committee to explain why he thought it was appropriate to meet with the Russian lawyer.

“We are talking about the wellspring and pride of our democracy ―– free and fair elections without foreign interference,” Schumer said on the Senate floor.