Donald Trump’s former national security adviser, John Bolton, has been asked to testify in the impeachment probe examining potential abuse of power by the president, U.S. media reported Wednesday.

Bolton, who resigned in September, has been called on to testify behind closed doors on Nov. 7, The Washington Post reported, as the White House seeks to limit cooperation with the probe by current and former administration officials.

Bolton was reported to have expressed alarm over secret administration efforts, including by Trump’s personal attorney, Rudy Giuliani, to pressure Ukraine to investigate Democrats, including potential Trump presidential rival Joe Biden.

Bolton’s lawyer and Democratic aides did not respond to requests for comment on the testimony notice or say whether Bolton had agreed to appear before investigators.