Neither a request for immunity from prosecution or a grant of immunity constitutes an admission of guilt. It does not mean the individual seeking immunity has committed a crime, only that he or she fears prosecution on the basis of their words, which absent immunity, they would not utter, instead invoking their Fifth Amendment privilege against self-incrimination.

As Michael Flynn’s lawyer said in a statement Thursday about his client’s offer to cooperate with congressional investigators in exchange for immunity from prosecution:

“He is now the target of unsubstantiated public demands by Members of Congress and other political critics that he be criminally investigated. No reasonable person, who has the benefit of advice from counsel, would submit to questioning in such a highly politicized, witch hunt environment without assurances against unfair prosecution”

But Flynn saw immunity differently in September 2016, when reports surfaced that aides around Hillary Clinton had been granted immunity by the government in exchange for talking freely to the FBI about Clinton’s private email server.

“When you are given immunity, that means you probably committed a crime,” Flynn, then a top campaign aide to Donald Trump, said on “Meet the Press.”

That same month then-Trump communications adviser Jason Miller said much the same thing.

“Revelations” that Clinton aides “were granted immunity from prosecution in Hillary Clinton’s email scandal shows this was without a doubt a criminal scheme,” he said.

Not to be outdone, Trump himself told a rally in Wisconsin around the same time that “The reason they get immunity is because they did something wrong, if they didn’t do anything wrong, they don’t think in terms of immunity.”

“If you are not guilty of a crime,” he said, “what do you need immunity for? Right.”

Trump posted this on Twitter Friday morning:

Mike Flynn should ask for immunity in that this is a witch hunt (excuse for big election loss), by media & Dems, of historic proportion!

— Donald J. Trump (@realDonaldTrump) March 31, 2017

Two congressional committees are now looking into whether any associates of President Trump may have coordinated with agents of the Russian government seeking to meddle in last year’s presidential election. The FBI is also investigating. The Trump administration has denied any such coordination.

Immunity for congressional testimony, which comes in a variety of forms with a variety of limitations and requires approval by a judge, has proven to be filled with complications.

“Use immunity,” for example, does not prevent a prosecutor from indicting a witness; it only stops prosecution using the testimony provided to the particular congressional committee.

“The witness can still be convicted for the crimes mentioned in the testimony on the basis of other evidence,” as Hannah Metchis Volokh wrote in a Georgetown Law Review article. She added:

“Keeping prosecutors and witnesses free from taint is especially difficult when immunized testimony is given in a highly publicized forum like a congressional hearing. Even if prosecutors can sometimes keep witnesses from hearing compelled testimony at criminal trials, the publicity surrounding a high-profile congressional investigation is almost impossible to avoid.”

The most famous example of a botched prosecution involving an immunity agreement was the case of Oliver North, the Reagan administration National Security Council staffer charged with crimes in connection with the 198os Iran-contra scandal.

An appeals court ultimately overturned North’s felony convictions, ruling in 1990 that while the special prosecutor in the case had not used North’s immunized testimony, witnesses in the case might have had their testimony “tainted” by exposure to the highly publicized and televised North congressional testimony.