In case you haven’t noticed, Special Counsel Robert Mueller takes lying very seriously. And, as we witnessed during Michael Flynn’s dramatic sentencing hearing, apparently so does Judge Emmet Sullivan. In fact, you would be hard-pressed to find a prosecutor or judge who does not.

To date, five defendants have been convicted of false statements in the special counsel’s Russia investigation – George Papadopoulos, Michael Flynn, Richard Gates, Alex van der Zwaan, and Michael Cohen. Jerome Corsi has indicated that the special counsel offered him a deal to plead guilty to false statements. In addition, Paul Manafort was convicted of conspiring to ask witnesses to lie, and Mueller’s team has ended a cooperation agreement with Manafort over allegations of lying.

Most recently, during Flynn’s halted sentencing hearing, Judge Sullivan pinned down Flynn on his admission guilt for lying and expressed his “disgust” and “disdain” for his behavior.

Critics and commentators such as Sen. Lindsay Graham and Rudy Giuliani have tried to downplay convictions for false statements as “process crimes,” suggesting that they are somehow less egregious than other violations of the law. Both Graham and Giuliani have records that belie this politically convenient argument. Giuliani oversaw a U.S. attorney’s office that prosecuted hundreds of so-called process crimes. And Graham voted to impeach President Bill Clinton for lies in judicial proceedings and, more recently, referred Christopher Steele to the Justice Department for alleged lies to the FBI.

To those who work in the criminal justice system, crimes based on lying are very serious, and for good reason.

First, lies impede an investigator’s quest for the truth. Our system of justice requires witnesses to tell the truth not just under oath on the witness stand, but also in their unsworn statements to law enforcement agents during investigations. When witnesses make false statements, the lies send investigators in the wrong direction, prevent them from confirming facts, and prolong or even derail the investigation. Anyone who complains about the duration of the special counsel’s investigation should be reminded that Mueller has charged 35 defendants in 18 months, despite proven lies by six key witnesses.

False statements offenses are charged in cases involving violent gangs, drug trafficking organizations, white collar crime, police corruption and, every other kind of case where people lie to protect themselves and others in their criminal organizations. In part these prosecutions hold accountable those who make it harder to solve crimes, and in part they deter others who might be tempted to do so. Imagine if there were no penalty for telling a lie – why would a witness to crimes by friends or associates ever tell an FBI agent the truth?

Second, in the context of counterintelligence investigations, lies can also compromise national security. Recall the congressional testimony of former acting Attorney General Sally Yates, who explained the significance of Michael Flynn’s lies. While serving as the National Security Advisor, Flynn had lied about his conversations with the Russian ambassador. Yates and the FBI knew that public statements about Flynn’s conversations contradicted what Flynn had actually said. As Yates testified, “To state the obvious, you don’t want your national security adviser compromised with the Russians.” A foreign adversary like Russia can use lies as leverage over government officials to coerce them into complying with its demands or else face exposure of the lies. For this reason, one of the questions that is asked during a background investigation for a security clearance is whether any matter in the person’s past could be used to blackmail him.

Cohen, President Donald Trump’s former lawyer, has now pleaded guilty to lying to the Senate intelligence committee about negotiations with Russia to build a Trump Tower in Moscow. Court documents indicate that Cohen discussed these negotiations with Trump and Trump’s family members, despite Trump’s public statements that he had nothing to do with Russia. If Cohen’s statements are accurate, they suggest that Trump and some of his family members may also be compromised with Russia to the peril of our national security.

Third, people lie even though they know that they are risking criminal penalties because telling the truth is worse. Prosecutors use evidence of lying as what is known as “consciousness of guilt.” The theory is that a person needs to lie only when he knows that the truth will get him into trouble. Cohen admitted that he lied to the Senate committee to “minimize links between the Moscow Project and Individual 1,” whom Cohen admitted is Trump.

What is it that all of these defendants close to Trump are lying to cover up? As Trump has tweeted, it is not a crime that he “lightly looked at doing a building somewhere in Russia.” So why would Cohen lie about it and risk criminal penalty? Two facts make it reasonable to infer that his secret relates to the core of Mueller’s mission to investigate links between the Trump campaign and Russian efforts to interfere with the election. First, an essential element of false statements is that the matter be “material” to the investigation. Innocent statements about a real estate development alone would not be material to the investigation into election interference. Charging this particular lie suggests that the Moscow Project relates somehow to election interference. In addition, to date, Mueller has handed off cases to U.S. Attorney’s offices that are not related to his core mission. The fact that he kept this one suggests that Cohen’s truthful statements are advancing the investigation into election interference. The cover-up may not be worse than the crime, but the cover-up raises suspicions that there is a crime.

It may be hard for political actors to understand why lies are so offensive to investigators and judges. While the truth always matters, politicians may exaggerate or engage in hyperbole or spin. But when people speak in the criminal justice system, the lies have consequences.

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