Former FBI chief Robert Mueller, who probed Trump's Russia ‘link’, dies at 81; Trump says ‘glad he’s dead'

Robert S. Mueller, who was nominated by George W Bush, began his 12-year tenure just one week before the 9/11 attacks.

Robert S Mueller, the former FBI director who transformed the enforcement agency into a terror-fighting force after the September 11, 2001, attacks, has died at the age of 81. Mueller was also the special counsel in charge of investigating ties between Russia and Donald Trump's presidential campaign.

Muller was also the special counsel in the Justice Department's investigation into whether the Trump campaign illegally coordinated with Russia to sway the outcome of the 2016 presidential election. (AP)

In a statement, Muller's family informed of his death, saying, "With deep sadness, we are sharing the news that Bob passed away" on Friday night, Associated Press reported.

Mueller, who was nominated by George W Bush, began his 12-year tenure just one week before the 9/11 attacks, serving across presidents of both political parties. He immediately overhauled the bureau's mission to meet the law enforcement needs of the 21st century. Democratic President Barack Obama later extended Mueller's appointment.

The 9/11 attacks resulted in a sudden shift in the bureau's top priority from tackling domestic crime to preventing terrorism, setting an almost unattainable standard for Mueller and the federal government.

Investigated Russia's ‘link’ to Trump campaign

After his tenure at the FBI, he was tapped as a special counsel for the Department of Justice to lead a probe between 2017-2019 into whether the Trump campaign illegally coordinated with Russia to sway the outcome of the 2016 presidential election.

During his tenure, FBI chief Mueller was known by some as "Bobby Three Sticks" due to his full name - Robert Mueller III. He reportedly displayed a patrician manner and sometimes a wooden personality.

Spread into a 448-page report, Mueller's Russia investigation has described a Russian campaign of hacking and propaganda to sow discord in the US. According to the report, it targeted 2016 Democratic presidential candidate Hillary Clinton and aimed at boosting Trump's popularity, the Kremlin's preferred candidate. Russia denied election interference.