Special counsel Robert Mueller could end up subpoenaing President Donald Trump if they can’t agree to an interview, but that could open up a fierce legal fight, according to a legal expert.

The Trump legal team is reportedly close to making a decision on whether to meet with Mueller for questioning. Mueller and his team have been looking into whether the president or his associates colluded with Russian interests during the 2016 election. Attorney Sol Wisenberg says if negotiations over a potential voluntary interview collapse, Mueller could issue a subpoena.

“Keep in mind that ultimately the Mueller people have the option of subpoenaing the president if he doesn’t agree to an interview,” Wisenberg told host Laura Ingraham on “The Laura Ingraham Show” Monday morning. “Then you have a real constitutional battle that Mueller is not at all certain to win, but at least it will go into the courts.”

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Wisenberg currently works as a partner with the law firm Nelson Mullins Riley & Scarborough LLP. He notably served as the deputy independent counsel during the political scandals involving then-President Bill Clinton and was selected by Judge Kenneth Starr to conduct grand-jury questioning of the former president.

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“We’re talking about presidential privilege at its highest,” Wisenberg said. “You’re talking about an interview with the president about acts he took in his official capacity as president. You’re talking about a fairly strong showing that Mueller would have to make to the court — basically that the evidence, he needs it, and that he can’t get it from anyone else, and that it is crucial to his investigation.”

Deputy Attorney General Rod Rosenstein appointed Mueller to lead the investigation last year after Attorney General Jeff Sessions recused himself from everything related to the collusion allegations. Republicans have expressed concern that the investigation may be biased against Trump on many different levels.

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The FBI also has faced questions over how it handled an investigation into former Secretary of State Hillary Clinton’s use of a private email server to conduct official diplomatic business. Several congressional committees have been demanding hundreds of thousands of documents related to the investigation.

Department of Justice Inspector General Michael Horowitz released a report June 7 detailing the results of his review of the email investigation. The report concluded that there were no signs of bias impacting decision-making, but critics have pointed to examples cited throughout the report and argued that they demonstrate pervasive bias.