Sen. Mark Warner (D-Va.) raised the alarm in a Friday evening statement, as Republican allies of President Donald Trump have pressed the Justice Department for details about a source believed to have aided the FBI and Special Counsel Robert Mueller’s probe into Trump campaign contacts with Russians.

“It would be at best irresponsible, and at worst potentially illegal, for members of Congress to use their positions to learn the identity of an FBI source for the purpose of undermining the ongoing investigation into Russian interference in our election,” Warner said. “Anyone who is entrusted with our nation’s highest secrets should act with the gravity and seriousness of purpose that knowledge deserves.”