Special counsel Robert S. Mueller III testifies on Capitol Hill in Washington before the House Judiciary Committee in Washington, July 24, 2019. Mueller, a former FBI director, died on Friday. He was 81. (AP Photo/Susan Walsh, file)

WASHINGTON (AP) — Robert S. Mueller III, the FBI director who transformed the nation’s premier law enforcement agency into a terrorism-fighting force after the Sept. 11, 2001, attacks and who later became special counsel in charge of investigating ties between Russia and Donald Trump’s presidential campaign, has died. He was 81.

“With deep sadness, we are sharing the news that Bob passed away” on Friday night, his family said in a statement Saturday. “His family asks that their privacy be respected.”

At the FBI, Mueller set about almost immediately overhauling the bureau’s mission to meet the law enforcement needs of the 21st century, beginning his 12-year tenure just one week before the Sept. 11 attacks and serving across presidents of both political parties. The cataclysmic event instantaneously switched the bureau’s top priority from solving domestic crime to preventing terrorism, a shift that imposed an almost impossibly difficult standard on Mueller and the rest of the federal government: preventing 99 out of 100 terrorist plots wasn’t good enough.

Later, he was 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 race. His investigation concluded that Russia interfered in the election on Trump’s behalf and that the Trump campaign welcomed the help, but Mueller and his team ultimately found insufficient evidence of a criminal conspiracy and did not make a prosecutorial decision about whether Trump had obstructed justice.

Mueller was maligned throughout the two-year investigation by Trump, who regularly derided it as a “witch hunt.” But the patrician Princeton graduate and Vietnam veteran who walked away from a lucrative midcareer job to stay in public service remained silent throughout the criticism, exhibiting an old-school, buttoned-down style that made him an anachronism during a social media-saturated era.

Trump posted on social media after the announcement of Mueller’s death: “Robert Mueller just died. Good, I’m glad he’s dead.” The Republican president added, “He can no longer hurt innocent people!”

Republican President George W. Bush, who nominated Mueller, said in a statement that he was “deeply saddened” by Mueller’s death and praised him for having “dedicated his life to public service” and for overhauling the FBI’s mission. Democratic President Barack Obama, who kept Mueller on even after his 10-year term had expired, called him “one of the finest directors in the history of the FBI” who saved “countless lives” after transforming the bureau.

“But it was his relentless commitment to the rule of law and his unwavering belief in our bedrock values that made him one of the most respected public servants of our time,” Obama added.

The FBI did not respond to a request seeking comment and current Director Kash Patel, a Trump loyalist, did not immediately note the death on social media. The FBI Agents Association cited Mueller’s “commitment to public service and to the FBI’s mission.”

The second-longest-serving director in FBI history, behind only J. Edgar Hoover, Mueller held the job until 2013 after agreeing to Obama’s request to remain on the job after the conclusion of his term.

After several years in private practice, Mueller was asked by Deputy Attorney General Rod Rosenstein to return to public service as special counsel in the Trump-Russia inquiry.

Mueller’s stern visage and taciturn demeanor matched the seriousness of the mission, as his team spent nearly two years quietly conducting one of the most consequential, yet divisive, investigations in Justice Department history. He held no news conferences and made no public appearances during the investigation, remaining quiet despite attacks from Trump and his supporters and creating an aura of mystery around his work.

All told, Mueller brought criminal charges against six of the president’s associates, including his campaign chairman and first national security adviser.

His 448-page report released in April 2019 identified substantial contacts between the Trump campaign and Russia but did not allege a criminal conspiracy. Mueller laid out damaging details about Trump’s efforts to seize control of the investigation, and even shut it down, though he declined to decide whether Trump had broken the law, in part because of department policy barring the indictment of a sitting president.

But, in perhaps the most memorable language of the report, Mueller pointedly noted: “If we had confidence after a thorough investigation of the facts that the president clearly did not commit obstruction of justice, we would so state. Based on the facts and the applicable legal standards, however, we are unable to reach that judgment.”

The nebulous conclusion did not deliver the knockout punch to the administration that some Trump opponents had hoped for, nor did it trigger a sustained push by House Democrats to impeach the president — though he was later tried and acquitted on separate allegations related to Ukraine.

The outcome also left room for Attorney General William Barr to insert his own views. He and his team made their own determination that Trump did not obstruct justice, and he and Mueller privately tangled over a four-page summary letter from Barr that Mueller felt did not adequately capture his report’s conclusions.