The FBI's special counsel's team is digging into the limits of President Donald Trump's pardon powers.

Because there is little precedent for governing presidential pardon powers, the special counsel, Robert Mueller, is likely to research state-level cases with elements that could be applied to the Russia investigation.

Experts say Mueller is taking unprecedented steps to ward off efforts by the White House to guard itself against the investigation.

Reports that the FBI's special counsel's team is researching President Donald Trump's pardon power are the latest indication that the Russia investigation is moving into unexplored terrain.

Robert Mueller, who was appointed special counsel in May after Trump fired the FBI Director James Comey, is tasked with examining Russia's interference in the 2016 US election, including whether Trump's campaign colluded with Moscow to tilt the race in his favor.

Bloomberg reported on Tuesday that Michael Dreeben, a seasoned prosecutor working with Mueller, was delving into past presidential pardons as the special counsel lays out his case — in particular, Dreeben is examining the limits of Trump's pardon power.

There is no federal precedent governing those limits, so Dreeben, a veteran Department of Justice attorney who has argued more than 100 cases before the Supreme Court, is most likely digging into state-level cases with elements that could be extrapolated as they relate to Trump's executive authority.

Like the president, governors also have the authority to grant pardons — and Renato Mariotti, a former federal prosecutor, told Business Insider that it was possible Dreeben was looking into state-level case law going back to the country's early years to find instances in which a court found a governor's pardon problematic.

"Dreeben's probably spending all day researching those cases, and he's probably going to find some analogy that Mueller can use" in case Trump issues a preemptive pardon or one that stymies the investigation, Mariotti said.

Mariotti added that the revelation was "really something" and one of the biggest developments in the investigation so far.

'At some point, a court might have to step in'

Donald Trump. AP Photo/Evan Vucci

Trump's pardon power became a subject of interest after The Washington Post reported in July that the president had asked his advisers whether he could pardon aides, family members, or possibly himself as the Russia investigation ramped up.

In addition to Trump's campaign, Mueller's investigation also encompasses several of the president's close associates, including his campaign chairman, Paul Manafort; his former national security adviser, Michael Flynn; his senior adviser and son-in-law, Jared Kushner; and his son Donald Trump Jr.

The Constitution grants the president very broad pardon powers relating to federal crimes — something Trump has pointed out.

"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 in July.

Trump's statements and actions indicate that "we have every reason to think that this president will not hesitate to use any means at his disposal to avoid scrutiny of himself, his family, and his inner circle," said Claire Finkelstein, a professor at the University of Pennsylvania Law School.

The president in August pardoned Joe Arpaio, the Arizona sheriff who was convicted of criminal contempt in July for violating the terms of a 2011 court order in a racial-profiling case. Arpaio was an early and ardent advocate for Trump on the campaign trail, and Trump told a crowd at a rally in Arizona shortly before granting the pardon that Arpaio would be "just fine."

"Imagine a circumstance where Trump pardons one of his associates in the Russia probe — that associate is then compelled to testify, but they refuse to do so, they're then held in contempt, and Trump pardons them again," Mariotti said. "At some point, a court might have to step in because the president would be defeating the lawful function of our criminal justice system."

Experts are also exploring whether Trump could pardon someone if doing so would fit into a pattern of obstruction of justice.

Finkelstein laid out a scenario in which parallels could be drawn to the circumstances surrounding Comey's firing in May.

James Comey. Drew Angerer/Getty Images

The White House initially said Comey was dismissed because of his handling of the investigation into former Secretary of State Hillary Clinton's use of private email, but Trump later told NBC's Lester Holt that "this Russia thing" had been a factor in his decision.

Comey also told the Senate Intelligence Committee in June that before firing him, Trump asked him to drop the Russia investigation and the FBI's inquiry into Flynn, who was forced to resign as national security adviser when it emerged that he had misled the vice president about his contacts with Russians during the transition period.

While the president has the power to fire the FBI director, he does not have the right to do so if his intent is to cover up an investigation into whether he or his associates committed misdeeds. Whether Trump had "corrupt intent" when he fired Comey is the basis of Mueller's obstruction-of-justice investigation.

"One could make a parallel argument here and say that if the president's motive in issuing a pardon is to avoid scrutiny of his or his associates' actions, that constitutes an overstepping of his constitutional authority," Finkelstein said.

There is, however, one key difference between the two scenarios: Trump's power to grant pardons is explicitly listed in the Constitution, while his power to fire officials is implicit in his right to govern as head of the executive branch.

'There's very little precedent for this'

Jon Michaels, a professor at the UCLA School of Law, said Tuesday that it was becoming clear that by probing the questions surrounding Trump's pardon power, Mueller's team was "being very strategic" and was "trying to preempt efforts that have the effect of insulating the administration from the investigations."

The lengths to which the special counsel's team is going to get ahead of the White House are unusual, said Jens David Ohlin, a vice dean at Cornell Law School who's an expert in criminal law.

"These definitely are not normal or typical prosecution strategies," he said. But, he added, "this is no usual investigation," and "there's very little precedent for this."

Mariotti agreed and said that in his long career as a federal prosecutor, he never investigated someone he thought the president might pardon in the middle of the investigation.

"Think of how rare that is," he said. "These are things we've never seen before. The pardon power is usually used years later, when someone's already served time. So it's unusual — very unusual — for a prosecutor to be considering these questions."

It's impossible to know how things would play out if Mueller faced off with Trump over Trump's authority to grant pardons, experts said, because of its unique and broad nature, and because it is written in the US Constitution.

"It would take a lot for a court to limit the power," Mariotti said. "But this is the sort of crazy situation where it just might happen."