The special counsel, Robert Mueller, is probing the limits of Trump's pardon power.

There are several unanswered constitutional questions regarding Trump's pardon powers Mueller's team is reportedly looking into, all of which have little precedent.

" I suspect that both sides are preparing for a possible showdown on these issues," said one legal expert.

Special counsel Robert Mueller's team is looking into whether there are any limits on President Donald Trump's pardon powers as the FBI investigates Russia's interference in the 2016 election.

Bloomberg reported on Tuesday that Michael Dreeben, a seasoned prosecutor working with Mueller, is delving into past presidential pardons as the special counsel lays out a legal strategy, to ensure that Mueller's case has a solid foundation and can stand up to possible appeals, the report said.

Trump's pardon power — and its possible limits — became a subject of interest after The Washington Post reported in July that the president asked his advisers if he could pardon aides, family members, and possibly himself as the Russia investigation picked up steam.

Constitutionally, the president's pardon powers are very broad as they relate to federal crimes, which Trump pointed out in July.

"While all agree the U. S. President has the complete power to pardon, why think of that when only crime so far is LEAKS against us.FAKE NEWS," Trump tweeted at the time.

So far, established constitutional limits on the president's pardon power prevent it from affecting impeachment proceedings and bar it from being applicable to state crimes.

But longtime federal prosecutor Renato Mariotti wrote Tuesday that it doesn't mean no other constitutional limits exist; merely that they have not yet been tested. Those possible limits are likely the questions Dreeben is looking into.

For instance, the special counsel's team may be examining how a pardon may affect witness testimony. Pardoning a witness voids their Fifth Amendment right not to testify, Mariotti noted. So "can a President pardon someone for a crime, and then pardon them for criminal contempt if they refuse to testify?" he asked. "Any limits on it?"

Jens David Ohlin, the vice dean at Cornell Law School and an expert on criminal law, said the issue of contempt is critical in the Russia investigation and that he would not be surprised if Mueller's team was actively researching it.

There are two types of contempt: criminal and civil. Civil contempt is typically imposed by a judge to induce compliance with his or her orders. For instance, if a witness refuses to testify and defies a judge's order, the judge may have the witness jailed until he or she complies. Criminal contempt is typically initiated by a prosecutor, requires a trial, and its purpose is to punish someone for past misconduct.

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If Trump uses his pardon power in either of those cases, it "is incredibly problematic and would aggrandize the executive branch's power" and compromise the judiciary's independence, Ohlin said.

But he emphasized that pardoning someone held in civil contempt would be "especially dangerous, and historically unprecedented, because it would effectively remove the court's authority to enforce its own judgments."

Mariotti laid out a number of other possibilities Dreeben may be looking into. "Another question he might research is whether the president can pardon a co-conspirator or someone who aided him in committing a crime," Mariotti wrote. "He also might ask if the President can pardon someone if the pardon would itself be part of a pattern of obstructing an investigation."

Mueller is reportedly building an obstruction-of-justice case against Trump centered around his decision to fire FBI director James Comey in May. At the time, Comey was spearheading the bureau's Russia investigation. The White House initially said Comey was dismissed because of his handling of the Clinton email investigation, but Trump later told NBC's Lester Holt that "this Russia thing" had been a factor in his decision.