PHILADELPHIA — The Justice Department has opened a criminal investigation into Philadelphia District Attorney Larry Krasner’s office and sent grand jury subpoenas in recent days in an inquiry centered on potential obstruction of justice, according to two sources with knowledge of the probe.

The request is the latest development in a high-profile saga centered on a post-conviction case that has led to a series of controversial revelations about Krasner’s office.

And according to the sources — who requested anonymity to discuss an ongoing investigation — the inquiry was opened in response to a memorandum from U.S. District Judge Paul S. Diamond, who said last month that federal prosecutors should investigate Krasner for potential conspiracy, obstruction of justice, or perjury over actions he and his office took in the matter pending in Diamond’s courtroom.

One of the subpoenas sought documents and emails connected to that underlying case, said one of the sources, and was broadly directed at the office rather than any potential witnesses or targets.

A spokesperson for the U.S. Attorney’s Office in Philadelphia declined to comment Tuesday on whether that office was involved in the investigation into Krasner. Spokespeople for the Department of Justice in Washington declined to comment.

Krasner said he could not respond to questions about any potential grand jury investigation because such cases, by law, are conducted in secret.

“I follow the rules,” he said.

Krasner, in court filings, has denied wrongdoing. And he has consistently said Diamond drew improper, unfair, and biased conclusions about his office’s actions in the case, calling the judge a “ringmaster” of a “circus,” and saying his interpretations of what happened were not only wrong, but sometimes “Orwellian.”

Diamond has defended his conduct, and recently denied a request by Krasner to remove himself from the case.

The controversy has stemmed from a post-conviction matter before Diamond in which a man named Dennis Johnson has sought to overturn his 2009 murder conviction.

Krasner’s office, for a time, agreed that Johnson’s conviction should be thrown out. But earlier this year, prosecutors tried to change course after discovering that the staffer who filed the brief backing Johnson’s position had made a series of mistakes.

After that discovery, those prosecutors and Krasner held discussions about what went wrong and how the office should move forward. And this summer, as Diamond called hearings to address similar questions, the prosecutors told the judge the DA had acted in ways they viewed as inappropriate — including, they said, by trying to pin all blame on the staffer who made the mistakes, saying they needed to “protect the office,” and threatening “consequences” for those who didn’t follow his orders.

Diamond said some of the allegations he heard — particularly those involving Krasner and one of his office’s supervisors, Matthew Stiegler — could amount to criminal wrongdoing.

Krasner has denied that, saying Diamond’s analysis “rests on nothing of substance.” And he said some of the prosecutors who spoke out against him — several of whom have since resigned — exhibited insubordination as they participated in the case, and were “legally incorrect” about disagreements they had with Krasner over how to handle the matter.

—Staff writer Ellie Rushing contributed to this article.

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