The US Justice Department has told former special counsel Robert Mueller he should limit his testimony before Congress tomorrow to discussing his public report on the Russia probe.
In a letter to Mr Mueller, Associate Deputy Attorney General Bradley Weinsheimer said his testimony "must remain within the boundaries of your public report because matters within the scope of your investigation were covered by executive privilege".
Mr Weinsheimer said: "These privileges would include discussion about investigative steps or decisions made during your investigation not otherwise described in the public version of your report."
In March, Mr Mueller completed his nearly two-year probe into Russian meddling in the 2016 election and possible obstruction of justice by US President Donald Trump.
The Justice Department released a redacted copy of his 448-page report in April.
A spokesman for Mr Mueller, Jim Popkin, said no one at the Justice Department, Congress or the White House would review Mr Mueller's statement before he delivers it tomorrow.
In back-to-back hearings before the House of Representatives Judiciary and Intelligence committees, Democrats are expected to try to get Mr Mueller to focus his testimony on specific examples of Mr Trump's misconduct.
The chairman of the Judiciary Committee, Democratic Representative, Jerrold Nadler, said on Sunday he believed there was "substantial evidence" that Mr Trump committed high crimes and misdemeanours.
Mr Nadler's comments are significant because evidence of such crimes would be required if Democrats pursue impeachment proceedings against the president.
Mr Trump has denied any wrongdoing and repeatedly slammed the Mueller investigation as a "witch-hunt".
Mr Mueller said in a statement on 29 May he would not go beyond what was in the report in any testimony to Congress.
The Mueller report laid out numerous contacts between Russian officials and Trump's campaign, but found no evidence of a criminal conspiracy.
It also gave examples of ten incidents in which Trump sought to hinder the investigation, but it did not draw any conclusions on whether Mr Trump obstructed justice.
Attorney General William Barr later concluded he did not see enough evidence to bring obstruction charges.