WASHINGTON — Matthew G. Whitaker, the acting attorney general, announced on Monday that the special counsel, Robert S. Mueller III, is wrapping up his investigation of Russian interference in the 2016 presidential campaign and whether the Trump campaign cooperated with Moscow’s operation.

“The investigation is, I think, close to being completed, and I hope that we can get the report from Director Mueller as soon as possible,” Mr. Whitaker said.

His impromptu remark, at the end of a news conference about an unrelated case, was a highly unusual confirmation of the state of the special counsel’s investigation. Justice Department policy ordinarily prohibits public comment on open criminal inquiries, and Mr. Mueller has refused to publicly discuss the investigation since he was appointed in May 2017 to oversee it.

Mr. Whitaker, who is considered intensely loyal to President Trump, said that he had been “fully briefed on the investigation” and that he looked forward to Mr. Mueller delivering a report.