The Justice Department’s decision to drop the criminal case against Michael T. Flynn, President Trump’s former national security adviser, even though he had twice pleaded guilty to lying to investigators, was extraordinary and had no obvious precedent, a range of criminal law specialists said on Thursday.

“I’ve been practicing for more time than I care to admit and I’ve never seen anything like this,” said Julie O’Sullivan, a former federal prosecutor who now teaches criminal law at Georgetown University…

“A pardon would have been a lot more honest,” said Samuel Buell, a former federal prosecutor who now teaches criminal law at Duke University…

No career prosecutors signed the motion. Mr. Shea is a former close aide to Mr. Barr. In January, Mr. Barr installed him as the top prosecutor in the district that encompasses the nation’s capital after maneuvering out the Senate-confirmed former top prosecutor in that office, Jessie K. Liu.