Two days before Robert Mueller testifies before Congress, the Justice Department has sent him a letter warning that he “must remain within the boundaries” of the public report on the Russia investigation.

Associate Deputy Attorney General Bradley Weinsheimer told the former special counsel that he was restricted in what he could say because the probe included matters of executive privilege—which allows the president to keep certain information secret.

“These privileges would include discussion about investigative steps or decisions made during your investigation not otherwise described in the public version of your report,” the letter said.

Weinsheimer said it was “standard practice” for DOJ witnesses to refuse to answer questions that touched on issues of privilege so the department can later find a way to provide answers to Congress while “protecting Executive Branch confidentiality interests.”

The letter came in response to a request by Mueller two weeks ago for guidance from the Justice Department on his Wednesday testimony before the House Judiciary and Intelligence committees.

Mueller has already said that he doesn't intend to stray from his report, which outlined 10 episodes of possible obstruction of justice by President Trump but did not declare whether he had committed a crime or impeachable offense.

Democrats on the committees are hoping to pry some new details from the famously tight-lipped former FBI director about Trump’s actions and his view of those actions.

The DOJ letter reiterated that the department deems Mueller’s testimony “unnecessary” but acknowledged that the decision to answer questions is his to make—while hammering down on the point that he should not disclose anything that isn’t in the report or Mueller’s May 29 public statement.

In that statement, Mueller appeared to contradict Attorney General William Barr by saying that he did not make a determination about whether Trump broke the law because DOJ policy prohibits indictment for a sitting president, and not necessarily because he did not think a crime had occurred.