A federal judge on Wednesday appointed a so-called special master to determine whether materials the F.B.I. seized from current and former Project Veritas employees can be used by prosecutors as evidence in their investigation into the apparent theft last year of a diary kept by President Biden’s daughter.

The ruling by Judge Analisa Torres of U.S. District Court came as a victory for Project Veritas, the conservative group that has found its conduct under scrutiny in recent weeks by the U.S. attorney’s office in Manhattan as it investigates how the diary kept by Ashley Biden made its way to Project Veritas in the weeks before Election Day last year.

The group asked the judge to appoint the special master to ensure that the authorities did not gain access to materials covered by attorney-client privilege and that the group’s First Amendment rights as a media organization were protected.

Despite ruling in Project Veritas’s favor, Judge Torres declined a request by the group to order the Justice Department to conduct a leak investigation into who provided information about the searches to The New York Times, which reported on them shortly after they occurred in early November.

In appointing the special master, Judge Torres acknowledged that federal prosecutors could conduct the review themselves “with utmost integrity.”

But she said the appointment of a special master was warranted because it was important both that the process be fair and that it appear to be fair.

“In light of the potential First Amendment concerns that may be implicated by the review of the materials seized from petitioners, the court finds that the appointment of a special master will ‘help to protect the public’s confidence in the administration of justice,’” the judge said.

Barbara S. Jones, a retired former U.S. District Court judge in Manhattan who now works as a private lawyer, will serve as the special master, Judge Torres said.

It is common for special masters to be appointed in high-profile investigations, where the authorities may have obtained materials that the government is often barred from having because they are protected by attorney-client privilege. A special master was appointed earlier this year to determine what evidence prosecutors can use that they obtained in a search of the home and office of Rudolph W. Giuliani, who served as President Donald J. Trump’s personal lawyer.

Federal prosecutors and F.B.I. agents have been investigating the role Project Veritas might have played in any theft of the diary. Project Veritas has acknowledged that it paid to obtain the diary but said that it had been assured by the individuals who sold it to the group that they had legally procured it.

Project Veritas has sought to portray itself as a journalistic organization whose First Amendment rights are being trampled over by an overzealous Justice Department. A lawyer for the group praised the ruling in a tweet, saying it was a sign that the group’s First Amendment rights were being protected.