Sen. Lindsey Graham (R-SC) came out of a Senate-wide briefing Thursday by the U.S. deputy attorney general with the impression that the federal probe into Russia’s interference in the U.S. election has entered criminal territory.

“The takeaway I have is that everything he said was that you need to treat this investigation as if it may be a criminal investigation,” Graham told reporters after the briefing from Deputy Attorney General Rod Rosenstein.

He cautioned that he didn’t get to ask Rosenstein to confirm that the investigation, which includes whether Donald Trump campaign officials colluded with Russian officials, was now a criminal one.

“It was a counterintelligence investigation before now. It seems to me now to be considered a criminal investigation,” he told reporters.

Graham, who sits on the Senate Judiciary Committee, argued that Rosenstein’s appointment of former FBI Director Robert Mueller as special counsel in charge of the probe would have the effect of limiting Congress’ ability to carry out parallel investigations.

“What does that mean for the Congress? I find it hard to subpoena records of somebody like Mr. Flynn who may be subject to a criminal investigation because he has a right not to incriminate himself,” he said, referring to Trump’s ousted national security adviser Michael Flynn.

Senate Intelligence Committee Chair Richard Burr (R-NC) had signaled earlier in the day that Flynn’s lawyers refused to honor a subpoena from his panel, before correcting himself. The subpoena requested documents related to any of Flynn’s dealings with Russia.

This post has been updated.