Robert Mueller might not be done just yet.

Democrats have demanded that the special counsel appear in person before Congress to testify about his 448-page report and explain his decision to leave open the question of whether President Donald Trump obstructed justice in the investigation over whether his campaign conspired with the Russians during the 2016 election. An hour before the redacted report became public on Thursday, Representative Jerrold Nadler, the chairman of the House Judiciary Committee, sent Mueller a letter formally requesting his testimony “no later than May 23.”

The request could open up a new front in the battle between congressional Democrats and the Department of Justice, which swiftly assumed ownership of Mueller’s report after the special counsel handed it in nearly a month ago. An appearance by Mueller before Congress would mark the first words he has uttered in public in the two years since Deputy Attorney General Rod Rosenstein appointed him to lead the probe into Russian interference and possible obstruction by the president. The special counsel was notably absent from Attorney General William Barr’s press conference on Thursday morning, ahead of the report’s release.