Maybe it's in the eyes - a certain gaze that is both shrewd and resolute, the look of a lawman who most likely has seen it all. And then some.
Those who have led the FBI are a special breed; that goes without saying.
The bureau has had 17 directors in the past century, some manning the post for a matter of weeks, some for years, one for decades.
Each has been tenacious, balancing the gut instinct of a cop with the exactitude of an investigator, the decisiveness of a trial attorney with the deliberation of a chess player - while employing the patience of Job. The FBI is, after all, is a federal agency.
The job of its director requires a kaleidoscope of virtues and street smarts. It is fueled by brute force, cunning, delicate instincts, discipline, diplomacy. There is the matter of mission: The job is not without a calling to serve public and president, to uphold good, vanquish evil and shore up the American way in all its permutations over the decades.
"We are a fact-gathering organization only. We don't clear anybody. We don't condemn anybody," J. Edgar Hoover once said.
If anyone, Mr. Hoover was entitled to summarize the agency's role, having served as its director from 1924 to his death in 1972, though his near five decades of service ultimately prompted the FBI to put a 10-year limit on the post's tenure.
However, he also had a stark fix on criminal nature. Mr. Hoover publicly concluded at one point that every lawbreaker he had ever encountered was a liar. Justice itself was "incidental to law and order," he said, and the effect of astute fingerprinting, stakeouts and apprehension couldn't compare to a most basic, very human influence.
"No amount of law enforcement can solve a problem that goes back to the family," Mr. Hoover said. "The cure for crime is not the electric chair, but the highchair."
Over the years, the rarified fraternity of FBI directors was drawn from a spectrum of specialties. Legal minds have been paramount: Eleven of the 17 directors had law degrees. Two were former U.S. attorneys; two were federal judges. Three were former military officers and seven former clandestine agents, either with the Secret Service or the FBI. One was a certified public accountant.
The challenges of the day - both criminal and cultural - dictated their duties.
Stanley Finch, the first director, took his post in the summer of 1908, tasked with defeating "white slave traffic," specifically, foreign women in the nation's brothels. Finch had suffragettes to thank, in part, for his appointment.
The primary reason "for establishing a separate office and title for the white slave traffic assignment" was the rallying cry of the woman's suffrage movement, notes an FBI document dating from the 1940s.
Finch's successor, Alexander Bruce Bielaski, was seven years on the job, 1912 to 1919, building the agency's resources - and emerged in 1919 to considerable personal drama. He was kidnapped in Mexico, escaped his captors with the ransom money in hand, then went on to work as an undercover Prohibition agent in a decoy speak-easy in Manhattan.
Acting Director William Allen was on the job just four months before William J. Flynn arrived. The burly former Secret Service agent took over from 1919 to 1921, heralded by Justice Department admirers as "the greatest anarchist chaser" in America.
Next in line, William J. Burns - also a former agent - enjoyed a piquant reputation in gossip columns and detective magazines as an international crime investigator.
"The flame-red hair and full mustache of his youth had faded, but not his seemingly limitless energy. He wore vested suits decorated with a gold watch chain and sported a black derby," noted one popular description of the time.
Burns eventually was asked to resign in 1924 for his entanglements in the Teapot Dome scandal.
It is Mr. Hoover, the agency's sixth director, who is best known to Americans. He was the face of the FBI for 48 years, babying and bullying the agency through several identities and eight White House administrations.
The son of civil servants and a champion debater even as a lad, Mr. Hoover was a native Washingtonian with pronounced convictions.
"His life-long guiding principles were formed early: he was convinced that middle-class Protestant morality was at the core of American values, and he harbored a deep distrust of alien ideas and movements that called those values into question," says a World Book Encyclopedia entry.
He was a driven achiever, working days at the Library of Congress and earning his law degree at night from George Washington University. At 22, he began work at the Justice Department and was made supervisor of a surveillance operation that netted 4,000 "alien Communists" in two years.
Mr. Hoover was offered the FBI director's job at age 29 - and the appointment became a very personal mission.
He set about turning his agency into a well-oiled machine by standardizing hiring practices and investigative techniques. In the early 1930s, some high-profile gangster arrests and new legislation that expanded FBI powers brought the agency considerable recognition as the likes of "Baby Face" Nelson and George "Machine Gun" Kelly were brought to justice by Mr. Hoover and his G-men - dramatically chronicled in newsreels and breathless press accounts.
"Hoover was famous for his successes in public relations, legend-building and image-making his Bureau into a Hollywood extravaganza, firmly entrenched as a mainstay of popular culture through films, comic strips, books, and carefully orchestrated publicity campaigns. The FBI and its director became dear to the hearts of the American people, and Hoover himself became a hero of almost mythic proportions," says the World Book Encyclopedia biography.
World War II and the 1950s set a different stage for Mr. Hoover, who pursued suspected Nazis and communists with zeal, closely allied with the House Un-American Activities Committee and such anti-communist politicians as Richard M. Nixon, then a California lawmaker, and Sen. Joseph McCarthy.
He made loyal friends. He also made fierce enemies, including the Kennedy family, according to many historic accounts. Few crossed Mr. Hoover, it was said, because he had "something" on them.
"Critics of Hoover argued - and continue to argue - that he went beyond law enforcement in these efforts, using so-called dirty tricks to undermine the reputation of persons he believed to be subversive," says a Legal Encyclopedia biography.
But he was thorough. Mr. Hoover kept files on the rich, the famous, the infamous - tracking such luminaries as John Lennon and Martin Luther King. When the FBI transferred Mr. Hoover's collection of confidential files to the National Archives in 2005 - 33 years after his death - the cache of more than 8 million documents included half a million pages devoted to civil unrest and five boxes devoted to Tokyo Rose, the World War II-era radio propagandist.
It was after his death that Mr. Hoover himself was investigated by emboldened researchers who claimed he had hid a multitude of lifestyles. They said he was gay, was a cross-dresser or had had a longtime relationship with a male associate. Others claimed the real love of Mr. Hoover's life was the beautiful actress Dorothy Lamour.
As with the Kennedy assassination, the truth may never be teased out from the tumult of opinions.
Then there is the agency itself - which has been influenced by 11 other directors since Mr. Hoover. They, too, wrestled with the criminal and cultural influences of their day, from Watergate to Sept. 11 to the advent of the Internet and a relentless 24/7 news cycle.
"In the end, if we in the FBI safeguard our civil liberties but leave our country vulnerable to terrorist attack, we have lost. If we protect America from terrorism but sacrifice our civil liberties, we have also lost. Every day, the men and women of the bureau must strike this balance," Director Robert S. Mueller III said at the National Press Club in May.