A federal judge is signaling that he may pursue perjury or contempt charges against former national security adviser Michael Flynn over his effort to abandon an earlier guilty plea to a charge of lying to the FBI.

The Justice Department moved last week to drop the prosecution of Flynn launched by Special Counsel Robert Mueller, but U.S. District Court Judge Emmet Sullivan threw a major stumbling block in front of that effort by announcing that he is appointing a former federal judge to advise the court on how to respond to the government’s motion.