Robert Mueller, the former special counsel appointed to investigate Russian interference in the 2016 presidential election and a longtime FBI director, died Friday at the age of 81, his family confirmed.

Mueller, a Marine Corps veteran and career law enforcement official, served as FBI director from 2001 to 2013, overseeing the bureau during a pivotal period that included the aftermath of the September 11 terrorist attacks and a major shift toward counterterrorism operations.

He later returned to public service in 2017 when the Justice Department appointed him as special counsel to lead an investigation into Russia’s efforts to influence the 2016 election and examine whether any coordination occurred with Donald Trump’s presidential campaign.

Over the course of nearly two years, Mueller’s team brought charges against multiple individuals, including former Trump associates and Russian nationals. The investigation resulted in several indictments and convictions, largely tied to financial crimes and false statements to federal investigators.

Mueller’s final report, released in 2019, concluded that Russia engaged in a broad effort to interfere in the election, including through cyber operations and social media activity. The report did not establish that the Trump campaign conspired with Russia, and it did not reach a definitive conclusion on whether Trump obstructed justice, instead presenting evidence on both sides of that question.

Following news of Mueller’s death, Trump reacted publicly, criticizing the former special counsel’s record and role in the investigation.

Before leading the Russia probe, Mueller held multiple senior roles within the Justice Department and was widely known in Washington for his reserved demeanor and long tenure in public service.

His death marks the passing of a figure closely tied to one of the most scrutinized federal investigations in recent political history. Further details about the circumstances of his death were not immediately released.