Lindsey Graham, who has served as the Republican senator for North Carolina since 2003, told reporters that during an all-senators briefing Deputy Attorney General Rod Rosenstein had hinted that the probe over Russia ties “may be a criminal investigation”.
While Mr Graham said had been unable to ask Mr Rosenstein directly, he added: “The takeaway I have is, everything he said was that you need to treat this investigation as if it may be a criminal investigation.”
Mr Graham said the appointment of Mr Mueller would prevent Congress' powers of investigation
The senator’s comments come after Mr Rosenstein sent shockwaves through Washington when he announced former FBI director Robert Mueller had been appointed special prosecutor to oversee the investigation into whether the Trump campaign had colluded with Russia to win last year’s presidential election.
Mr Mueller was appointed after James Comey, who originally oversaw the investigation, was sacked from his role as FBI director by President Trump.
Speaking to reporters about the meeting, Mr Graham said: “The general consensus was it was a good decision to pick a special counsel, a lot of confidence in Mr Mueller – I think the shock to the body is it’s now considered a criminal investigation.”
Mr Mueller was appointed special prosecutor by Deputy Attorney General Rod Rosenstein
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Mr Graham said he thinks "Congress' ability to conduct investigations of all things Russia has been severely limited, probably in an appropriate fashion”.
The senator added that Mr Mueller’s appointment would make it harder for Congress to subpoena records of people who could be subjects of the investigation.
President Trump declared angrily on Thursday he was the victim of a witch hunt after a reporter asked him whether reports he had asked Mr Comey to drop his investigation into disgraced national security adviser Mike Flynn were true.
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Addressing the appointment of Mr Mueller as special prosecutor, the President said: “I respect the move. But this entire thing has been a witch hunt.
“And there is no collusion between, certainly, myself and my campaign – but I can always speak for myself – and the Russians, zero.”
Trump decried a "witch hunt" when asked about questions about the Russia scandal yesterday
Earlier in the day, the Mr Trump accused his predecessor, President Barack Obama and former Democratic rival Hillary Clinton of committing “illegal acts” during Mrs Clinton’s campaign last year.
He tweeted: “With all the illegal acts that took place in the Clinton campaigned Obama Administration, there was never a special counsel appointed!”