(FILES) L-R: US President Barack Obama (L), Attorney General Eric Holder, Secretary of Homeland Security Janet Napolitano and Director of the Federal Bureau Investigation (FBI) Robert Mueller stand during the National Anthem at the National Peace Officers Memorial Service, an annual ceremony honoring law enforcement who were killed in the line of duty in the previous year, at the US Capitol in Washington, DC, May 15, 2013. Mueller, the former FBI director who led a politically explosive investigation into President Donald Trump, has died aged 81, US media reported Saturday. (Photo by Saul LOEB / AFP via Getty Images)

(FILES) Former Special Counsel Robert Mueller arrives to testify about his Report on the Investigation into Russian Interference in the 2016 Presidential Election during a House Select Committee on Intelligence hearing on Capitol Hill in Washington, DC, July 24, 2019. Mueller, the former FBI director who led a politically explosive investigation into President Donald Trump, has died aged 81, US media reported Saturday. (Photo by SAUL LOEB / AFP via Getty Images)

(FILES) Robert Mueller (R) being named the new director of the Federal Bureau of Investigation (FBI) by US President George W. Bush (L) at a White House Rose Garden ceremony, on July 5, 2001 in Washington, DC. Mueller, the former FBI director who led a politically explosive investigation into President Donald Trump, has died aged 81, US media reported Saturday. (Photo by MIKE THEILER / AFP via Getty Images)

(FILES) US President Barack Obama listens to his introduction by FBI Director Robert Mueller before speaking to employees at FBI Headquarters in Washington, DC, April 28, 2009. Mueller, the former FBI director who led a politically explosive investigation into President Donald Trump, has died aged 81, US media reported Saturday. (Photo by Saul LOEB / AFP via Getty Images)

(FILES) US President Barack Obama applauds outgoing Federal Bureau of Investigations (FBI) director Robert Mueller (L) in the Rose Garden at the White House in Washington, DC on June 21, 2013 as he nominates Jim Comey to be the next FBI director. Mueller, the former FBI director who led a politically explosive investigation into President Donald Trump, has died aged 81, US media reported Saturday. (Photo by Nicholas KAMM / AFP via Getty Images)

(FILES) Former Special Counsel Robert Mueller listens as he testifies before the House Select Committee on Intelligence hearing on Capitol Hill in Washington, DC, July 24, 2019. Mueller, the former FBI director who led a politically explosive investigation into President Donald Trump, has died aged 81, US media reported Saturday. (Photo by SAUL LOEB / AFP via Getty Images)

(FILES) Former FBI Director Robert Mueller, special counsel on the Russian investigation, leaves following a meeting with members of the US Senate Judiciary Committee at the US Capitol in Washington, DC on June 21, 2017. Mueller, the former FBI director who led a politically explosive investigation into President Donald Trump, has died aged 81, US media reported Saturday. (Photo by SAUL LOEB / AFP via Getty Images)

(FILES) Special Counsel Robert Mueller speaks on the investigation into Russian interference in the 2016 Presidential election, at the US Justice Department in Washington, DC, on May 29, 2019. Mueller, the former FBI director who led a politically explosive investigation into President Donald Trump, has died aged 81, US media reported Saturday. (Photo by MANDEL NGAN / AFP via Getty Images)