WASHINGTON (Reuters) - When Robert Mueller was leading a U.S. Marine Corps platoon in combat in Vietnam, Donald Trump was finishing college and about to go to work for his father, a New York City landlord, having received military draft deferments including for a diagnosis of bone spurs in his heels.

FILE PHOTO: U.S. Special Counsel Robert Mueller arrives to make his first public comments on his investigation into Russian interference in the 2016 U.S. presidential election at the Justice Department in Washington, U.S., May 29, 2019. REUTERS/Jim Bourg

The contrasts between the two men - the voluble real estate developer turned politician and the no-nonsense former FBI director - are many, and could be on full display on Wednesday when Mueller testifies to two congressional panels about his long investigation of Trump through the lens of Russia’s 2016 U.S. election interference.

Mueller, 74, served for 22 months as a special counsel appointed by the Justice Department, completing his inquiry in March. He has faced withering criticism from the Republican president, who has accused Mueller of conflicts of interest and called his team of lawyers “thugs” and a “national disgrace.”

Since Mueller’s 448-page investigative report was released by the Justice Department in redacted form on April 18, it has been widely reprinted and became a best-selling book, while Trump, 73, has launched regular Twitter attacks on it and its author.

On Monday, Trump tweeted: “Highly conflicted Robert Mueller should not be given another bite at the apple. In the end it will be bad for him and the phony Democrats in Congress who have done nothing but waste time on this ridiculous Witch Hunt. Result of the Mueller Report, NO COLLUSION, NO OBSTRUCTION!”

Mueller, the architect of the modern FBI and a long-serving federal prosecutor, has largely kept out of view since the report’s release aside from a nine-minute appearance before reporters on May 29 at the Justice Department in which he said the inquiry had been conducted in “a fair and independent manner.” During that event, Mueller said, “The report is my testimony. I would not provide information beyond that which is already public in any appearance before Congress.”

He took no questions and left.

The weeks following have seen the Democrats who control the U.S. House of Representatives dig into some of the unanswered questions in the report as they struggle to decide whether to pursue the impeachment process set out in the U.S. Constitution for removing a president from office.

FROM BRONZE STAR TO FBI

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.

He studied law at the University of Virginia and launched into a career of private practice and public service. In 2001, Mueller, a Republican, was named to head the FBI by Republican President George W. Bush and was unanimously confirmed by the Senate. He started work a week before the Sept. 11, 2001 attacks on the United States.

Democratic President Barack Obama extended his service. By the time Mueller left the position in 2013, his tenure was exceeded in length only by J. Edgar Hoover’s 48-year stint.

Mueller was credited with transforming the FBI into an agency centered on protecting national security in addition to law enforcement, putting more resources into counterterrorism and improved cooperation with other U.S. agencies.

In his long career, Mueller had stints in private practice but preferred government work. In the 1990s, he left a major law firm to take a low-level job in the U.S. Attorney’s office in the District of Columbia, specializing in homicide cases at a time when the U.S. capital experienced a high murder rate.

Mueller 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 organized crime boss John Gotti and the investigation into the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland.

He was named special counsel in May 2017, again receiving bipartisan praise in Washington. He took over the FBI’s investigation of Russian election meddling after Trump fired FBI Director James Comey.

Mueller’s special counsel report said the investigation found insufficient evidence to conclude that Trump and his 2016 campaign engaged in a criminal conspiracy with Russia despite the numerous contacts. The report did not reach a conclusion on whether Trump committed the crime of obstruction of justice but did not exonerate him. Attorney General William Barr, a Trump appointee, subsequently concluded the president had not committed obstruction of justice.

Longstanding Justice Department policy bars criminal charges against a sitting president.

After negotiations about a presidential interview with the special counsel’s team, Mueller let Trump give written responses to questions about whether his campaign conspired with what U.S. intelligence agencies described as Russian hacking and propaganda aimed at dividing Americans and boosting Trump’s candidacy. Trump provided the written answers in November 2018.

Mueller’s special counsel investigation in the Russia matter resulted in charges against 34 people, including some key figures in Trump’s campaign, and three Russian entities.

“And as set forth in the report,” Mueller told reporters in May, “after that investigation, if we had confidence that the president clearly did not commit a crime, we would have said so.”