BATON ROUGE, La. — An FBI agent in New Orleans raised legitimate concerns that the Justice Department is either “unable or unwilling” to self-police ethical lapses within its ranks, a federal judge said in a ruling Thursday.

U.S. District Judge Kurt Engelhardt refused to order the release of a 31-page letter that FBI Special Agent Michael Zummer wrote about his investigation of a former Louisiana district attorney accused of trading sex for leniency.

But the judge wrote that he shares Zummer’s concerns about the Justice Department and found the agent’s correspondence to be “particularly interesting (and troubling, to say the least).”

Last month, Engelhardt sentenced former St. Charles Parish District Attorney Harry Morel to three years in prison for obstructing the federal investigation. The judge received the letter from Zummer two days before the sentencing hearing.

During an extraordinary news conference after Morel’s guilty plea earlier this year, state and federal authorities accused Morel of being a sexual predator who solicited sex from at least 20 women in exchange for favorable treatment from his office.

But they haven’t charged Morel with any sex crimes.

Engelhardt said Zummer’s letter provided information about internal communications at the FBI and the Justice Department “as well as his disagreement with various prosecutorial decisions.”

The judge isn’t sealing Zummer’s correspondence or barring its disclosure, but he said the docket for Morel’s case isn’t the “appropriate venue” for releasing the information. Federal prosecutors opposed the release of Zummer’s letter.

However, the judge said he shares the agent’s concerns that the Justice Department is “either unable or unwilling to self-police lapses of ethics, professionalism and truthfulness in its ranks.” His ruling doesn’t elaborate on the nature of Zummer’s concerns.

Federal prosecutors had asked Engelhardt to seal Zummer’s correspondence.

“Publicly filing these submissions, while not revealing the exact contents of the privileged portions, would still reveal information related to confidential communications that (Agent) Zummer was not authorized to disclose to the Court or public,” they wrote.

Engelhardt was sharply critical of how the Justice Department handled its prosecution of five former New Orleans police officers charged in deadly shootings on a bridge in Hurricane Katrina’s aftermath. A jury convicted the former officers in 2011, but Engelhardt threw out the verdicts amid allegations of prosecutorial misconduct, including prosecutors’ anonymous online postings.

The commenting scandal led to the resignations of two veteran prosecutors and former U.S. Attorney Jim Letten, although Letten wasn’t implicated in the postings on a newspaper’s website.

FBI spokesman Craig Betbeze said Thursday that his office had no comment on the judge’s ruling in Morel’s case. A spokeswoman for the U.S. attorney’s office in New Orleans didn’t immediately respond to requests for comment.