Robert Mueller, the former FBI director who led a politically explosive investigation into Donald Trump, has died aged 81.

His death triggered a gloating response from the US president.

The New York Times reported Mueller’s death, citing a family statement, but did not specify a location or cause.

Trump responded quickly on Truth Social, writing: “Robert Mueller just died. Good, I’m glad he’s dead. He can no longer hurt innocent people!”

Mueller led the FBI for 12 years, starting days before the September 11, 2001, Al-Qaeda attacks on the United States, during which time he built up the bureau’s counterterror mission.

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 Trump’s presidential campaign conspired with Russia to get him elected.

An obituary published by the Washington Post said Mueller had spent more than four decades in law enforcement - as line prosecutor, US attorney and FBI chief, developing a reputation as a stickler for detail who savored methodical investigation and nearly always got his man.

He became the longest-serving leader of the FBI since J. Edgar Hoover, and he guided the bureau under Presidents George W. Bush and Barack Obama through its transformation from a domestic crime-fighting agency to a global intelligence organisation focused on the war against terrorism.

He built a reputation for nonpartisan rectitude and stone-faced reserve but suddenly became a political target in his last major role on the national stage - as special counsel investigating Russian interference in the 2016 presidential election that brought Trump to office.

Donald Trump gloated on social media, saying it was "good" that Robert Mueller had died. Photo / Getty Images

Attacked by the right

Mueller found himself excoriated from the right as a symbol of what Trump and his supporters viewed as a witch hunt designed to bring down the president. At the same time, his investigation was welcomed by the left as an injection of integrity, an impartial reminder of a time when a consensus on the facts was politically achievable.

Mueller maintained a public silence through the 22 months of his Russia investigation, which yielded indictments leading to convictions of a Trump campaign chairman, a deputy campaign manager, a national security adviser and one of his personal attorneys.

In his report, released in April 2019, Mueller concluded that “the Russian government perceived it would benefit from a Trump presidency and worked to secure that outcome”, and that the Trump campaign “expected it would benefit electorally from information stolen and released through Russian efforts”.

But the investigation “did not establish that members of the Trump Campaign conspired or coordinated with the Russian government” - a conclusion that Trump and his supporters cited as evidence that the probe had been a partisan fishing expedition.

Trump regularly denounced the Mueller investigation as a “hoax”, “a total disgrace” that amounted to “treason”. In 2020, he issued pardons to national security adviser Michael Flynn, Trump’s political adviser Roger Stone and several others convicted in the probe.

- Agence France-Presse, with additional reporting by the Washington Post