The investigation into alleged Russian meddling in the 2016 U.S. election appears to be considered criminal, an important Republican senator said Thursday after he received a briefing from the deputy U.S. attorney general.

"It was a counterintelligence investigation before now. It appears to me now to be considered a criminal investigation," Sen. Lindsey Graham, R-S.C., said after Deputy Attorney General Rod Rosenstein briefed all 100 senators.

Graham's comments came after the Justice Department on Wednesday appointed former FBI Director Robert Mueller as special counsel on the investigation.

When ex-FBI Director James Comey publicly confirmed the investigation in March, he referred to it as a counterintelligence investigation. The probe includes looking into any possible links between Trump campaign associates and Russia.