WASHINGTON (CBSNewYork/AP) – Special Counsel Robert Mueller raised the possibility of issuing a subpoena for President Donald Trump if he does not agree to answer questions about the Russia investigation.

That’s according to Trump’s attorney.

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Trump attended the swearing-in of Secretary of State Mike Pompeo Wednesday amid new signs of escalating tension between the president’s legal team and Special Counsel Robert Mueller.

One of the president’s former lawyers tells the Associated Press Mueller raised the idea of a subpoena during a meeting in early March to force the president to testify under oath.

In January, the president told reporters he was interested in talking with the special counsel.

“I would love to do that, and I’d like to do that as soon as possible,” Trump said.

In a Wednesday morning Tweet, the president once again called the investigation a “witch hunt,” dismissing claims of collusion as a “hoax” and obstruction of justice as a “setup and trap.”

A subpoena has always been an option for the special counsel but to use it on a sitting president would be an extraordinary move.

If Mueller issued one, the president could fight it in court or he could invoke his fifth amendment right against self-incrimination.

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“The bare minimum for Mueller is going to be to get some answers from Trump under oath,” said former Department of Justice official Harry Litman. “No prosecutor would settle for less.”

Meanwhile, Deputy Attorney General Rod Rosenstein, who oversees Mueller’s investigation, pushed back against reports that Republican lawmakers were drafting articles of impeachment against him.

“I think they should understand by now the Department of Justice is not going to be extorted,” Rosenstein said.

Republican members of the House Freedom Caucus say impeachment would be used against Rosenstein as a last resort.

Those members say impeachment would only be considered if Rosenstein doesn’t comply with requests for information pertaining to the Russia investigation.

The White House announced Wednesday that attorney Ty Cobb will retire at the end of the month.

Cobb has been the point person for the White House regarding the Russia probe.

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(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)