The appointment of Robert S. Mueller III as special counsel to conduct an investigation related to a sitting president roiled Washington around the start of President Trump’s first term.

Robert S. Mueller III, the former FBI director who led the investigation into ties between Russia and the Trump campaign, has died at the age of 81. He was also known for transforming the FBI into a terrorism-fighting force after the September 11 attacks.

Robert S. Mueller III, ex-FBI director and key figure in post-9/11 terrorism efforts, died at 81.