More than a year ago, I wrote a series of columns that made the point that former national security adviser Michael Flynn had been set up by the FBI and may have committed no crime. I repeated these arguments in several interviews, and this led the professional Donald Trump bashers, who care more about “getting” the president than protecting civil liberty, to attack me for “inventing” legal principles that assist the White House.

Well it now turns out that I was right all along. Recent revelations in this case prove that the FBI set up Flynn and sprung a perjury trap in order to get him to lie. But if he did lie, some might ask, what is the difference how they got him? A crime is a crime, regardless of what the FBI does to make him commit it, is it not? The answer to this is not necessarily.

Lying to the FBI is not a federal crime unless the lie is actually material to an investigation. Thus, if the FBI is investigating terrorism, and the subject falsely denies an extramarital affair, his lie would not be material. So is a lie material when the FBI already knows the truth because it has it on tape? Is it material if the only reason the FBI asked the question was not to get the subject to provide truthful information but to get him to lie?

Charles Breitel, one of the most distinguished appellate judges in modern history, answered these important questions within the context of grand jury testimony. Breitel explained, “The primary function of the grand jury is to uncover crimes and misconduct in public office for the purpose of prosecution. It is not properly a principal aim of the grand jury, however, to ‘create’ new crimes in the course of its proceedings.”

He continued, “Thus, where a prosecutor exhibits no palpable interest in eliciting facts material to a substantive investigation of crime or official misconduct and substantially tailors his questioning to simply extract a false answer, a valid perjury prosecution should not lie.”

Another federal court, indeed in the same district where Flynn allegedly committed his crime, has also made a similar point within the context of a legislative investigation, stating that “extracting testimony with a view to a perjury prosecution” is not a “valid legislative purpose.” These judges are right, although other judges have disagreed with them. What they said is just as relevant within the context of questions by the FBI.

Beyond the strict legalities lies the broader questions of policy. Is it the proper role of law enforcement to conduct criminal “morality tests” to determine whether citizens will lie when given the opportunity by FBI investigators? I believe the answer should be no. The function of law enforcement is to uncover past crimes. It is not to provide citizens the opportunity to commit new crimes by testing their veracity.

There may be extraordinary circumstances, such as terrorism prevention, that justify the use of highly questionable tactics but, absent such risks, FBI agents and prosecutors should not deliberately provide citizens the opportunity to commit federal crimes in order to prosecute them, make political arguments, or turn them into government witnesses.

When they question suspects, officials should not ask questions whose answers they know, for the sole purpose of finding whether the suspect will lie. If they must ask such questions, untruthful answers should not be material to the investigation, because the officials already knew the truth. That is the civil liberty approach to proper law enforcement.

Flynn has pleaded guilty and he is now seeking to withdraw that plea. The judge in his case should allow him to do so, based on the new information that should have been disclosed before he made his plea decision. More is at stake than his personal future, although that is important.

The integrity of the FBI is also on the line. It is the proper role of the courts to check the prosecutorial excesses of law enforcement officials. Allowing Flynn to withdraw his guilty plea would serve as an effective check on the highly questionable tactics that were used by the FBI on him.

Alan Dershowitz, professor emeritus at Harvard Law School, served on the legal team representing President Trump during the Senate impeachment trial. He has written more than 40 books, including his newest one, “Guilt by Accusation: The Challenge of Proving Innocence in the Age of Me Too.”