WASHINGTON (Reuters) - Twelve days after being named special counsel to investigate Russia’s role in the 2016 U.S. presidential election, Robert Mueller delivered a speech to his granddaughter’s high school graduating class at Tabor Academy in Marion, Massachusetts.

FILE PHOTO: Robert Mueller, as FBI director, listens during a U.S. Senate Judiciary Committee oversight hearing about the Federal Bureau of Investigation on Capitol Hill in Washington, June 19, 2013. REUTERS/Larry Downing/File Photo

In this rare public appearance in May 2017, Mueller did not bring up President Donald Trump or the investigation, but offered a clear message stressing the importance of honesty and integrity.

“You could be smart, aggressive, articulate, indeed persuasive, but if you are not honest, your reputation will suffer,” Mueller said. “And once lost, a good reputation can never, ever be regained. The saying goes: If you have integrity, nothing else matters, and if you do not have integrity, nothing else matters.”

On Friday, Mueller handed in the long-awaited report on his investigation.

Mueller, a 74-year-old decorated Vietnam War veteran and former FBI director known for his tough, no-nonsense leadership style, has faced unremitting attacks by Trump on his integrity as the Republican president has tried to discredit the investigation and the special counsel himself. Trump declined to sit for an interview with the special counsel’s team.

Mueller has remained silent about the inquiry but has spoken loudly through court filings and the indictments of 34 people, including several key Trump aides as well as Russian intelligence officers and three Russian companies.

Mueller, a longtime Republican, was named by the Justice Department’s No. 2 official, Rod Rosenstein, to take over the Russia investigation after Trump fired FBI Director James Comey, whose agency had led the probe. Mueller has looked into whether Trump’s campaign conspired with Russia and whether the president has unlawfully tried to obstruct the investigation.

Trump, facing political peril from the inquiry, has used Twitter, campaign-style speeches and comments to news media to assail Mueller, accusing him of running a politically motivated, “rigged witch hunt;” going “rogue;” surrounding himself with “thugs” and having conflicts of interest.

“It’s all a big hoax,” Trump said on Friday.

Mueller has been a fixture in American law enforcement for decades and is considered the architect of the modern FBI, serving as its director from 2001 to 2013. He was first appointed to the post by Republican President George W. Bush, then his appointment was extended by Bush’s successor, Democrat Barack Obama.

Mueller took over as Federal Bureau of Investigation director a week before the Sept. 11, 2001, attacks on the United States by al Qaeda militants that killed about 3,000 people. By the time Mueller left the position, his tenure was exceeded only by J. Edgar Hoover’s 48-year stint.

‘BOBBY THREE STICKS’

Mueller was known by some as “Bobby Three Sticks” because of his full name - Robert Mueller III - a moniker that belies his formal bearing and sober approach to law enforcement.

He was credited with transforming the premier U.S. law enforcement agency after Congress and an independent government commission determined the FBI and CIA had failed to share information before the Sept. 11 attacks that could have helped prevent them. Mueller revamped the FBI into an agency centered on protecting national security in addition to law enforcement, putting more resources into counterterrorism investigations and improving cooperation with other U.S. agencies.

He put his career on the line in 2004 when he and Comey, then the deputy attorney general, threatened to resign when White House officials sought to reauthorize a domestic eavesdropping program that the Justice Department had deemed unconstitutional. The two rushed to a Washington hospital and prevented top Bush aides from persuading an ailing Attorney General John Ashcroft, recovering from gall bladder surgery, to reauthorize the surveillance program.

Comey succeeded Mueller as FBI director in 2013.

After graduating from Princeton University, Mueller served in the Marine Corps during the Vietnam War, leading a rifle platoon and receiving commendations including the Bronze Star. His became a federal prosecutor in 1976 and remained in public service until his FBI retirement, with the exception of a couple of short stints with law firms.

He became a U.S. assistant attorney general in 1991 and was a key player on high-profile federal prosecutions such as the 1992 convictions of former Panamanian leader Manuel Antonio Noriega and New York Mafia boss John Gotti and the investigation into the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland.

Mueller’s Russia investigation already has yielded a series of guilty pleas and a conviction in the only trial held to date.

Trump’s former campaign chairman Paul Manafort was convicted on eight charges and pleaded guilty to two others, receiving a 7-1/2-year prison sentence. Trump’s former national security adviser Michael Flynn, former personal lawyer Michael Cohen and former campaign aides Rick Gates and George Papadopoulos have entered guilty pleas. Longtime Trump adviser Roger Stone has pleaded not guilty to charges.

The big question is whether Mueller will present evidence of criminal conduct by Trump himself. Such findings could prompt the Democratic-controlled House of Representatives to begin the congressional impeachment process laid out in the U.S. Constitution for removing a president from office for “treason, bribery, or other high crimes and misdemeanors.”

While Trump has hammered away at Mueller, others have defended the special counsel’s integrity, including some formerly associated with the president such as former White House attorney Ty Cobb.

“I think,” Cobb said in an ABC News podcast interview, “Bob Mueller is an American hero.”