WASHINGTON — A lawyer Michael T. Flynn recently hired said on Monday that she would most likely need a security clearance to review classified materials that were separate from the documents that prosecutors for the special counsel, Robert S. Mueller III, had already handed over in the case.

The lawyer, Sidney Powell, made the unusual disclosure during a hearing in Federal District Court despite prosecutors’ insistence that they included no classified information in the evidence they shared with the defense. Her remarks heightened speculation that Ms. Powell — who has pushed conspiracy theories about the special counsel’s investigation and sells anti-Mueller T-shirts on her website — may be preparing to accuse the government of wrongdoing in Mr. Flynn’s case.

“There is other information,” she said, but she declined to elaborate.

One of the prosecutors, Brandon Van Grack, said he did not know what she was referring to. “There’s nothing that the government produced that’s classified,” Mr. Van Grack said.

Ms. Powell did not return a telephone message seeking further comment. Another lawyer working with her declined to comment. President Trump and his allies have long contended that Mr. Flynn and others were illegally targeted by the F.B.I. and intelligence community as part of a broad conspiracy to undermine the president.