A federal judge asked federal prosecutors Tuesday to investigate Philadelphia District Attorney Larry Krasner, saying he may have obstructed justice and potentially committed other crimes in his handling of a post-conviction case.

The memorandum by U.S. District Judge Paul S. Diamond Jr. represents an extraordinary turn of events in the controversy around Krasner’s Conviction Integrity Unit. It was issued as part of a case involving a man seeking to overturn his second-degree murder conviction.

Krasner’s office has backed the effort to exonerate the man, Dennis Johnson. But prosecutors assigned to the case have said that it was poorly handled and that Krasner ordered them to “protect the office” after the court began to examine their actions.

Diamond ruled that Krasner and his deputy, Matthew Stiegler, could no longer participate in the case. He said there is evidence that they may have sought to obstruct justice, as well as worked to persuade four prosecutors in their office to “urge the court to accept a false narrative,” which would “amount to criminal conspiracy.”

“I have thus reluctantly concluded that Philadelphia’s highest law enforcement official and one of his top assistants may have violated the law,” Diamond said in the memo. “In these circumstances, with the issuance of this Memorandum, I refer the DAO, Mr. Krasner, and Stiegler to this District’s United States Attorney’s Office for investigation and other appropriate action.”

Krasner pushed back against the judge’s memo.

“I respectfully and vigorously disagree with the court’s findings of fact and conclusions of law,” he said in a statement. “I look forward to appealing to the higher court as soon as the law allows. This remains a pending matter, which limits for now my and my office’s ability to respond in the media.”

A spokesperson for the U.S. Attorney’s Office for the Eastern District of Pennsylvania declined to comment.

Krasner, a former civil rights attorney known for suing the police, took over the district attorney’s office in 2018. Since he won re-election for the second time last year, his office has faced growing scrutiny over its handling of post-conviction cases.

The Pennsylvania attorney general, as well as state and federal judges, has criticized Krasner’s office for pushing to overturn murder convictions without proper reviews.

Over the summer, the Pennsylvania Supreme Court ordered the state attorney general to review any case in which Krasner’s office was seeking to overturn a conviction.

Krasner has defended his office’s handling of the cases.