The man who investigated former President Bill Clinton at the height of the real estate scandal Whitewater and the sexual allegations involving Monica Lewinsky praised a controversial federal investigation.

Ken Starr, who served as an independent counsel during the Clinton presidency, said Robert Mueller, serving as special counsel to the U.S. Department of Justice, has done good work prosecuting Russian interference in the 2016 election.

Starr defended Mueller’s integrity as a nonpartisan prosecutor, and said President Donald Trump would be unwise to fire him because doing so could set off extraordinary controversy and potentially lead to the appointment of another, similar prosecutor.