Paul Callan is a CNN legal analyst, a former New York homicide prosecutor and currently is counsel at the New York law firm of Edelman & Edelman PC , focusing on wrongful conviction and civil rights cases. Follow him on Twitter @paulcallan . The opinions expressed in this commentary are his own.

Though Flynn served for only 24 days as the president's national security adviser, he was a close confidant of Mr. Trump during the election campaign. He was so close to the nominee that he was vetted as a possible candidate for the vice presidency.

The position of national security adviser to the president, is an office of substantial importance to the nation. Previous occupants have included such luminaries as Walter Rostow, Zbigniew Brezinski, Henry Kissinger, Colin Powell and Condoleezza Rice.

The office, which was established in 1947, has been touched by scandal -- during the Reagan administration when two national security advisers, Robert McFarlane and John Poindexter, faced criminal charges relating to the Iran-Contra controversy and later, when Sandy Berger, a former adviser to President Bill Clinton, pled guilty to illegally removing documents from the National Archives after his term ended. (Poindexter's conviction was overturned on appeal and McFarlane was among those who received a presidential pardon.)

Flynn's plea to a single count of making false statements to the FBI brings dishonor to the post once again. His cooperation with the office of special counsel Robert Mueller minimizes the possibility that he will face the five-year maximum penalty possible under the statute.

Mueller's assignment under the direction of Deputy Attorney General Rod Rosenstein is to investigate "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump and...any matters that arose or may arise directly from the investigation."

Clearly, General Flynn's lying to FBI investigators would fall into the category of a matter arising from the investigation. A more interesting question is suggested by the nature of the false statement that federal prosecutors outline in the document filed in federal court Friday. They accuse Flynn of urging the Russians to refrain from responding to the sanctions that had been imposed on Russia by the Obama administration in retaliation for Russian interference in the election, presumably until Trump was sworn in as the new president.

What Mueller may really be looking for here is proof of a "quid pro quo" promise to ease up on sanctions to reward the Russians for assisting the Trump campaign in trashing Hillary Clinton during the presidential election. If Flynn could provide evidence of such an election-tampering deal made with knowledge of the President, impeachment proceedings would be inevitable.

By any reasonable account, Mueller is moving closer to the President, having now brought criminal charges against his former national security adviser and his former campaign chairman, Paul Manafort. Of course the Trump administration will assert that the Flynn indictment is of no moment because he served as national security adviser for such a short period of time before the President fired him.

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Mueller's investigators, of course, will remember that the FBI director fired by Trump has said that before his ouster the President asked him to drop the investigation of Flynn, pressed for James Comey's loyalty and sought assurances that Trump was not himself under investigation. Trump has forcefully denied the accuracy of Comey's statements, including the allegation that he sought an end to the investigation of Flynn.

Mueller will use Flynn to carefully examine the question of whether the President's action in firing Comey constituted obstruction of justice, which could be cited by the House as an impeachable offense. It is logical to assume that Flynn may have had conversations with the President about Comey's ongoing investigation, given his close relationship with Mr. Trump.

The special counsel has substantial leverage to compel cooperation in that General Flynn faces a potential five-year jail sentence under the lying to the FBI count and the special counsel also could move to indict Flynn's son on charges related to Flynn's business, if he really wants to bring the hammer down.

As the Flynn investigation moves closer to the White House, the President will undoubtedly say he is happy that the investigative ordeal will soon end with his total exoneration. Many in the White House may worry that the special counsel's investigation could end in an entirely different manner -- with an early end to the controversial President's term.