WASHINGTON — The Justice Department Monday warned former special counsel Robert Mueller that his upcoming congressional testimony should not stray from the written conclusions outlined in the 448-page final report of the two-year investigation into Russian election interference.

"Any testimony must remain within the boundaries of your public report because matters within the scope of your investigation were covered by executive privilege, including information protected by law enforcement...and presidential communications privileges," Associate Deputy Attorney General Bradley Weinsheimer wrote in a two-page letter to Mueller two days before his scheduled testimony before the House Judiciary and Intelligence committees.

Weinsheimer also cautioned that Mueller should abide by terms that the former special counsel discussed in May during his first public appearance following the report's delivery earlier this spring.

"Specifically, that the information you discuss during testimony appears in, and does not go beyond, the public versions of your March 22, 2019 (final) report to the attorney general or your May 29 public statement," Weinsheimer said.

Earlier Monday, Mueller spokesman Jim Popkin said the former FBI director would not veer from his written conclusions. He added that the former special counsel intended to deliver an opening statement that is expected to closely track his first public remarks about the case in May when he defended the investigation he supervised, and refused to clear President Donald Trump of criminal wrongdoing, asserting that bringing obstruction charges was "not an option" because of Justice Department policy against prosecuting a sitting president.

In a brief televised statement in May, Mueller recounted his report's overall findings, saying Russia launched a "concerted" effort to interfere with the 2016 election. "There were multiple, systematic efforts to interfere in our election, and that allegation deserves the attention of every American," he said.

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At that time, Mueller said the inquiry into Russia's efforts was one of "paramount importance," so investigators took seriously efforts by Trump and others to thwart their work. He said the Justice Department's Office of Legal Counsel prohibited the prosecution of a sitting president, and his team of prosecutors was bound to follow that rule.

Mueller said that if prosecutors had confidence that the president clearly didn't commit a crime, "we would have said that."

"The Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing," Mueller said then, describing the department's rationale for why a president cannot be prosecuted. He did not directly identify that process, but he was referring to the daunting political exercise of impeachment.

While Mueller had hoped that his May statement would be his last, his Wednesday appearances before the House Judiciary and Intelligence committees were secured by a subpoena.

Popkin said Mueller would make clear Wednesday that he was testifying because he had been subpoenaed to do so.

The Justice Department letter also underscores the forced nature of Mueller's testimony.

"As the attorney general has repeatedly stated, the decision to testify before Congress is yours to make in this case, but the department agrees with your stated position that your testimony should be unnecessary under the circumstances," Weinsheimer wrote.

"The department generally does not permit prosecutors such as you to appear and testify before Congress regarding their investigative and prosecutorial activity."

Some Democratic lawmakers believe Mueller's public testimony, even if only a dry recitation of the inquiry's most damaging findings, could kick-start support for impeachment proceedings against Trump.