Former FBI Director Robert Mueller, who served as special counsel investigating Russian interference in the 2016 presidential election, died Friday at age 81.

Mueller's family had previously disclosed to The New York Times in August that he had been diagnosed with Parkinson's disease. The circumstances surrounding his death were not immediately specified.

Mueller served as FBI director from 2001 to 2013, leading the bureau through the September 11 attacks and the subsequent restructuring of federal law enforcement priorities. He was appointed by President George W. Bush and his tenure was extended by President Barack Obama.

In May 2017, Mueller was appointed as special counsel to investigate Russian interference in the 2016 presidential election and potential coordination with the Trump campaign. His investigation resulted in multiple indictments and convictions of Trump associates and Russian nationals before concluding in March 2019.

President Trump responded to news of Mueller's death with criticism on social media, expressing satisfaction and launching attacks on the former FBI director's record and investigation.

Mueller previously served as a federal prosecutor and held various Justice Department positions throughout his career before leading the FBI.