Days after the FBI raided the hotel room and offices of Michael Cohen, President Trump’s attorney and fixer, the president granted a full pardon to I. Lewis “Scooter” Libby, who in 2007 was convicted of perjury, making false statements to the FBI and obstruction of justice in connection with a special counsel investigation. The timing of the pardon immediately prompted speculation that Trump wasn’t just concerned with Libby — he was dangling the ultimate get-out-of-jail-free card to keep Cohen from sharing incriminating evidence with investigators.

Trump dismissed the idea of a pardon for Cohen in a news conference a few weeks later. But the idea that he would use the presidential pardon power to attempt to limit the impact of special counsel Robert Mueller’s investigation isn’t mere conjecture. The president’s lawyer reportedly raised the possibility of pardons for Michael Flynn and Paul Manafort, two former Trump aides who have been indicted (Manafort) or pleaded guilty (Flynn) in the Russia investigation, and the idea has been proposed by Trump supporters and conservative activists.

The president has broken with so many norms that there’s a temptation to label almost everything Trump does as unprecedented. In this case, however, the record is a little more complicated. If Trump pardoned someone caught up in the Russia probe, it actually wouldn’t be the first time that pardons were used to absolve defendants in a special counsel investigation. According to our analysis of independent and special counsel probes since 1979, more than half of the investigations that resulted in criminal charges saw pardons for at least one person implicated, often — but not always — granted by the president under whose administration the investigation proceeded. But it would be unprecedented to issue a pardon so early in an investigation. All of the pardons granted in conjunction with special counsel investigations were issued at least five years after the investigation began, and a couple of the pardons were even issued months or years after the investigation ended.

According to legal experts, an early pardon could expose Trump to obstruction of justice charges for interfering with an ongoing criminal probe or fuel impeachment proceedings. And it’s possible that such a pardon might even violate the Constitution. “The timing makes a huge difference,” said Katy Harriger, a professor of politics and international affairs at Wake Forest University and an expert on special counsel investigations. “It could be a big problem for Trump if he appears to be using the pardon to keep people from sharing information that would be personally damaging to him.”

Of course, Trump hasn’t pardoned anyone involved in the Mueller probe. But looking at how pardons have been used in response to past special counsel investigations helps us understand how far other presidents have been willing to go — and where Trump might cross a line.

The pardon power gives presidents the virtually unlimited ability to grant clemency to people at any point in the legal process — even before they’ve been charged or convicted of a crime. And when faced with investigations that embroil their close associates, past presidents haven’t hesitated to use the pardon power as a corrective or even a challenge to special prosecutors like Mueller.

Many of these pardons, though, occurred in the late stages of an investigation or after it had ended — and often just as a president was leaving office.

As part of an infamous raft of pardons granted on his last day in the White House, Bill Clinton pardoned four people implicated in the Whitewater investigation — including his former business partner, who had served jail time for refusing to cooperate with investigators. At the same time, he pardoned his former housing secretary, Henry Cisneros, who had been investigated and fined for lying about payments to a former mistress. And he also pardoned six other people who were convicted as part of a separate independent counsel investigation into corruption within the Department of Agriculture during the Clinton presidency that was widely perceived as a waste of time and resources. (A seventh figure in the Espy investigation had already been pardoned the month before.)

The Espy pardons “weren’t as controversial as they might have been because the investigations were over and there was a lot of concern about fairness,” Harriger said. “There was a sense that these people were minnows who got caught in pursuit of the big fish.”

One president, George H.W. Bush, did use his pardon power to effectively end an independent counsel investigation. On Christmas Eve in 1992, Bush pardoned six former government officials, including former Defense Secretary Caspar Weinberger, who were defendants in the ongoing probe into the Iran-Contra scandal. The pardons were granted weeks before Weinberger was slated to be tried on charges that he lied to Congress about his knowledge of the Reagan administration’s sale of weapons to Iran and efforts to support right-wing “Contras” who were rebelling against the socialist Nicaraguan government. It was likely that Bush himself would have been called as a witness in Weinberger’s trial (in his role as vice president, Bush had kept a detailed diary as the scandal unfolded that the independent counsel subsequently accused him of withholding during the investigation).

John Q. Barrett, a former prosecutor on the Iran-Contra team and current professor at St. John’s University Law School, said the pardons abruptly terminated the investigation’s second phase, which had been mired in political controversy. “Our perception was that Bush and his advisers clearly had a negative, critical attitude toward our office’s work, and this was their attempt to shut us down,” Barrett said. With Weinberger suddenly free from prosecution, the investigation ended soon after.

There are parallels between the Iran-Contra pardons and a hypothetical pardon in the Russia investigation — including an adversarial relationship between the president and the special counsel and the possibility that the president might be implicated — but the circumstances are different in several key ways. By December 1992, the Iran-Contra investigation had been dragging on for six years, and Bush was poised to leave office. “There was public fatigue around the investigation and virtually no political consequences for Bush, since Clinton was about to take office and wanted to turn the attention to his agenda,” Barrett said.

Mueller’s investigation, in contrast, is only entering its second year and has shown no sign of slowing down. In modern history, at least, there’s no precedent for a president pardoning someone involved in a special counsel probe this early in the investigation.

It’s possible, of course, that Trump could follow in past presidents’ footsteps and wait until the Russia investigation has run its course before issuing pardons. This might be the most strategic course of action, according to Barrett, since a pardoned defendant could still provide valuable information to the special counsel as a witness and would no longer be able to use the Fifth Amendment to avoid self-incrimination. “If Trump pardons Manafort, Cohen, whomever — the next day, that person is entirely available for Mueller to pump dry,” Barrett said. “And if they lie or refuse to testify, they’ll be committing new crimes beyond what they’ve been pardoned for.”

Meanwhile, a pardon that appears to be designed to obstruct or end the Russia investigation could create additional political or legal troubles for Trump. Gerald Ford’s unpopular pardon of Richard Nixon — which may have cost him re-election — is one cautionary tale. It’s not clear whether the president can be charged with a crime like obstruction of justice while in office, but that doesn’t mean a pardon can’t cause political problems. Jed Shugerman, a law professor at Fordham University, said that it could be grounds for impeachment. And although a Trump pardon would be unlikely to be challenged in this way, Shugerman believes that self-interested pardons could violate a section of the Constitution that says the president will ensure that the country’s laws are “faithfully executed” — which could be interpreted as a legal obligation against self-dealing.

Trump has already proved willing to use his pardon power in controversial ways. Last summer, he pardoned Joe Arpaio, a former sheriff in Arizona who had been convicted of contempt of court related to his hard-line immigration enforcement tactics, prompting some legal scholars to speculate about whether Trump was testing the power’s limits. But even though using the pardon to free Cohen, Manafort or someone else implicated in Mueller’s investigation might be tempting, such a move could ultimately create more problems for Trump than it would solve.