Robert Mueller, the former FBI director who investigated ties between the Trump campaign and Russia, has died at the age of 81.

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

His family said in a statement on Saturday: “It is with deep sadness that we announce the news of Bob’s death” on Friday evening. “His family requests that their privacy be respected.”

At the FBI, Mueller almost immediately set about reforming the Bureau’s mission to meet the needs of 21st-century law enforcement, beginning his 12-year tenure just one week before the September 11 attacks, and serving through heads of both political parties. He was nominated by Republican President George W. Bush.

He watches: Robert Mueller testifies before Congress

This catastrophic event immediately shifted the office’s top priority from solving domestic crimes to preventing terrorism, a shift that imposed a difficult, almost impossible standard on Mueller and the rest of the federal government: Preventing 99 out of 100 terrorist plots was not good enough.

Later, he was a special counsel in the Justice Department’s investigation into whether the Trump campaign illegally coordinated with Russia to influence the outcome of the 2016 presidential race. Mueller was an aristocratic Princeton graduate and Vietnam veteran who left a lucrative job mid-career to remain in public service, and whose old-fashioned style made him an anachronism during an era saturated with social media.

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

The FBI did not immediately respond to a request for comment.

A second worked as an investigator for a sitting president

Mueller, the second-longest-serving director in FBI history, behind J. Edgar Hoover, held the position until 2013 after agreeing to Democratic President Barack Obama’s request to remain in office even after his 10-year term ended.

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

Mueller’s tough appearance and taciturn demeanor matched the seriousness of the mission, as his team quietly spent nearly two years conducting one of the most important, but contentious, investigations in the history of the Justice Department. He did not hold any press conferences or appear in public during the investigation, remaining quiet despite attacks by Trump and his supporters and creating an aura of mystery around his work.

Finally, Mueller brought criminal charges against six of the president’s aides, including his campaign chairman and senior national security adviser.

Read more: Robert Mueller agrees to testify before Congress regarding the Russia investigation

His 448-page report, released in April 2019, identified significant contacts between the Trump campaign and Russia but did not allege a criminal conspiracy. Mueller offered damaging details about Trump’s efforts to control, and even shut down, the investigation, though he declined to decide whether Trump broke the law, in part because of a department policy that prohibits indicting a sitting president.

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

The ambiguous conclusion did not deal the death blow to the administration that some Trump opponents had hoped for, nor did it lead to a sustained push by House Democrats to impeach the president — though he was later tried and acquitted in separate allegations related to Ukraine.

The outcome also left room for Attorney General William Barr to insert his own views. He and his team had made their own determination that Trump had not obstructed justice, and he and Mueller disagreed in particular over a four-page summary letter from Barr, which Mueller felt did not adequately express the damaging conclusion of his report.

Read more: Robert Mueller’s full statement on the Russia investigation

Mueller caused Democrats to cringe during an upcoming congressional hearing on his report when he gave terse, one-word answers and appeared uncertain in his testimony. Often, he seemed to hesitate about the details of his investigation. This was not the leadership performance many expected from Mueller, who had a good reputation in Washington.

Over the following months, Barr made clear his disagreements with the foundations of the Russia investigation and moved to dismiss Mueller’s false statements lawsuit against former National Security Advisor Michael Flynn, even though the investigation ended with a guilty plea.

Mueller’s tenure as special counsel marked the high point of his career in government.

Transforming the FBI into a national security agency

His time as FBI director was defined by the September 11 attacks and their aftermath, giving the FBI sweeping new surveillance and national security powers to confront the rising Al Qaeda network, disrupt plots, and get terrorists off the street before they could act.

It was a new model of policing for the FBI, which had long been accustomed to investigating crimes that had already occurred.

When he became FBI director, “I expected to focus on areas that were familiar to me as a prosecutor: drug cases, white-collar criminal cases, violent crimes,” Mueller told a group of lawyers in October 2012.

Instead, “we had to focus on long-term strategic change. We had to strengthen our intelligence capabilities and modernize our technology. We had to build on strong partnerships and forge new friendships, both here at home and abroad.”

In response, the FBI transferred 2,000 of the 5,000 total agents in the bureau’s criminal programs to Homeland Security.

In hindsight, the transformation was successful. At that time, there were problems, Mueller said. In a speech near the end of his term, Mueller recalled “those days when we were being attacked by the media and hammered by Congress; when the Attorney General was not happy with me at all.”

Among the cases: The Justice Department’s inspector general found that the FBI circumvented the law to obtain thousands of phone call records for terrorism investigations.

Read more: Trump declares “case closed” after Robert Mueller’s comments

Mueller decided that the FBI would not engage in abusive interrogation techniques for suspected terrorists, but this policy was not effectively deployed for nearly two years. In an effort to move the FBI to a paperless environment, the bureau spent more than $600 million on two computer systems — one that was two and a half years behind schedule, and another that was only partially completed and had to be scrapped after consultants declared it outdated and riddled with problems.

For the country’s top law enforcement agency, it has been a difficult journey through rugged terrain.

But there have been many successes as well, including foiling terrorist plots and headline-grabbing criminal cases like the one against conman Bernie Madoff. The Republican also gained an apolitical reputation in office, and nearly resigned in a conflict with the Bush administration over a surveillance program that he and his successor, James Comey, considered illegal.

He famously sided with Comey, then deputy attorney general, during a dramatic hospital confrontation in 2004 over federal wiretapping rules. The two men sat at the bedside of ailing Attorney General John Ashcroft to prevent Bush administration officials from making a final push to obtain Ashcroft’s permission to reauthorize a secret wiretapping program without a warrant.

In an extraordinary vote of confidence, Congress agreed, at the request of the Obama administration, to extend Mueller’s term in office for two years.

A Marine who served in Vietnam before becoming a prosecutor

Mueller was born in New York City and grew up in an affluent suburb of Philadelphia.

He holds a bachelor’s degree from Princeton University and a master’s degree in international relations from New York University. He then joined the Marine Corps, serving for three years as an officer during the Vietnam War. He led a rifle platoon and was awarded a Bronze Star, a Purple Heart, and two Navy Commendation Medals. After his military service, Mueller earned a law degree from the University of Virginia.

Mueller became a federal prosecutor and enjoyed the work of handling criminal cases. He rose rapidly through the ranks of the U.S. Attorneys’ offices in San Francisco and Boston from 1976 to 1988. Later, as head of the Justice Department’s Criminal Division in Washington, he oversaw a host of high-profile prosecutions that achieved victories against such diverse targets as Panamanian dictator Manuel Noriega and New York crime boss John Gotti.

Read more: Robert Mueller says he will leave the Justice Department

In a mid-career shift that shocked his colleagues, Mueller gave up his job at a prestigious Boston law firm to join the homicide division of the U.S. Attorney’s Office in the nation’s capital. There, he became immersed as a senior lawyer in a large number of unsolved drug-related murder cases in a city rife with violence.

Mueller was driven by his career-long passion for the painstaking work of building successful criminal cases. Even as head of the FBI, he delved into the details of investigations, some major cases and others less important, sometimes surprising agents who suddenly found themselves on the phone with the director.

“Management books will tell you that as the head of an organization, you must focus on the vision,” Mueller once said. But “for me, there were and still are areas where one needs to be very personally involved,” especially regarding “the terrorist threat and the need to know and understand the roots of that threat.”

Two terrorist attacks occurred at the end of Mueller’s reign: the Boston Marathon bombing and the shooting at Fort Hood in Texas. Both weighed heavily on him, as he admitted in an interview two weeks before his departure.

“You sit with the families of the victims, you see the pain they’re going through, and you always wonder if there was something more” that could have been done, he said.