Veteran trial lawyer Beth Wilkinson is helping guide U.S. District Judge Emmet Sullivan as a federal appeals court questions his plan to probe the U.S. Justice Department's decision to dismiss the case against former Trump national security adviser Michael Flynn despite his admission he lied to the FBI.

Sullivan refused to immediately dismiss the charge against Flynn, a retired Army general who served for just weeks in Trump's White House, and the U.S. Court of Appeals for the D.C. Circuit is now weighing whether the longtime Washington federal trial judge overstepped his authority.

The appeals court set a June 1 deadline for Sullivan to respond to a petition from Flynn's lawyers that seeks the immediate dismissal of the case. Sullivan, meanwhile, has appointed an outside lawyer, John Gleeson, a former federal judge in Brooklyn, to make arguments against the Justice Department's bid to ditch the prosecution. Flynn pleaded guilty in 2017 to lying to the FBI about his communications with the Russian ambassador to the U.S., but he later hired new lawyers to help him withdraw his acknowledgement of wrongdoing.

Wilkinson, a co-founder of the litigation boutique Wilkinson Walsh, confirmed Saturday she had been retained by Sullivan, who has served on Washington's federal trial court since 1994. The Washington Post first reported Wilkinson's role advocating for the judge.

Wilkinson has long been in the spotlight in Washington legal circles and beyond as a successful defense-side trial lawyer advocating for major U.S. companies. She left the law firm Paul, Weiss, Rifkind, Wharton & Garrison in 2016 to start the boutique, which has offices in Washington, New York and Los Angeles.

She has frequently taken on high-profile assignments. In 2018, she was hired by Brett Kavanaugh, then a U.S. Supreme Court nominee, to help shepherd him through confirmation proceedings at which he had been accused of a decades-old sexual misconduct allegation. Kavanaugh, who denied the claim, was confirmed to the high court.

More recently, Wilkinson was retained by Summer Zervos in a suit in New York state court that accuses President Donald Trump of lying in his denials that he did not grope and kiss her without consent in 2007. The case is pending.

In the D.C. Circuit now, Wilkinson is counsel to Cheryl Mills, a former aide to then-Secretary of State Hillary Clinton, in a dispute over a deposition in a public-records case. One of Sullivan's colleagues on the bench, U.S. District Judge Royce Lamberth, said the plaintiff—the conservative advocacy group Judicial Watch—could depose Clinton and Mills.

Sullivan's move to appoint Gleeson, now a partner at Debevoise & Plimpton, to oppose the Justice Department's move to dismiss was seen by some observers as an assertion of independence of the courts as a co-equal branch of government. Still others said Sullivan had assumed too powerful a role, and that he should have dismissed the case at the Justice Department's request.

Legal scholars are divided over how much power Sullivan holds to scrutinize the Justice Department's charging decisions. Federal rules do not allow prosecutors to unilaterally dismiss any criminal case. They need "leave of court" before a charge can be withdrawn.

At one time, Flynn was prepared to be punished, but he agreed to further cooperate with the special counsel's Russia investigation in an effort to secure a non-jail sentence. During Flynn's first scheduled sentencing, Sullivan raised the prospect that Flynn might go to prison for lying to federal agents.

Timothy Shea, the U.S. attorney for the District of Columbia and a former close adviser to Barr, asked Sullivan to dismiss the case against Flynn on the ground that prosecutors no longer believe there was a strong basis to justify the FBI's 2017 interview that is central to the false-statements claim. Former Justice Department officials have disputed the government's claim there was no ground to interview Flynn.