This story is the last in a series focusing on the FBI as the agency hits the 100-year mark.
WASHINGTON (CNN) -- At precisely 100 years of age, the FBI is learning new tricks. It has no choice.
Robert Mueller has led the FBI in a dramatic shift in focus since September 11, 2001.
Hardly spry, the massive crime-fighting agency of 30,000 agents and support staff is slowly, sometimes grudgingly, transforming itself into an intelligence-driven entity able to adapt to rapid technological change and savvy enemies.
Nobody understands the challenges of the new direction better than the man who has manned the bureaucratic battleship since the week before the terrorist attacks of September 11, 2001.
"In this world of globalization, in this world of technological advances, every 18 months the threats will change and we have to be agile enough to address those threats as they do change," FBI Director Robert Mueller told CNN earlier this month.
"We've made huge progress since September 11 in reorienting the FBI to address terrorism and cyber crime and counterintelligence and some of the threats of the future," he added.
The FBI has a wide-ranging portfolio of law enforcement responsibilities, but the unquestioned priority is the prevention of another terrorist attack. Watch Mueller discuss progress in anthrax probe »
That's a mind-boggling shift from the sometimes glamorous pursuit of the world's worst criminals. For decades, physically fit, well-trained agents with a hint of a swagger paraded bad guys into federal courts and jails, capturing the imagination of filmmakers and youngsters who wanted to become G-men.
Under the tight control of long-time director J. Edgar Hoover, whose name still graces the FBI headquarters building in Washington, the image was honed, and crime-fighting legends were born and magnified.
Those images seem destined to be replaced by pictures of modern-day specialists at computer screens sifting data to pinpoint terrorists, spies and cyber criminals. The running battle between criminals and crime-fighters has largely gone high-tech. Check out the FBI, then and now »
FBI critics and civil libertarians constantly fret about the dangers to individual privacy as the FBI dives into its new mission.
"The FBI's overzealousness in response to the 'red scare' of the '50s, '60s and '70s was counterproductive, but after 9/11 the lessons were quickly forgotten," said former FBI agent Mike German, who now works for the ACLU.
German, a 16-year veteran of the FBI, is among voices sharply criticizing FBI leaders for willingly adopting an aggressive high-tech intelligence stance -- potentially at the expense of privacy rights. German and other critics see it as a throwback to the abuses of the past.
"If it's by electronic means rather than shoe leather, I don't see the distinction," German said. "The pendulum is not yet swinging back, and we continue to lose ground on civil liberties."
The FBI, by its own admission, was slow to go high-tech, and is feverishly playing catch up. Belatedly, internal FBI communication networks have gradually improved.
"We are continuously refreshing our technology to stay on the cutting edge. We've still got a way to go but we've made great progress," Mueller said.
The FBI's most persistent critic on Capitol Hill for years has been Sen. Charles Grassley, an Iowa Republican who's planned speech today notes the Bureau's century of service while chastising it for its failures.
"For years, I've been a watchdog of the FBI's propensity to retaliate against whistle-blowers, the Bureau's unwillingness to cooperate with other agencies, and its inability to update its technology system. I hope on its 100-year anniversary, the FBI will turn a new leaf and correct these problems to create a better, safer century ahead," Grassley's text says.
Despite criticism, FBI agents from 56 field offices across the United States and more than 70 outposts abroad continue to chase terrorists, spies, murderers, drug traffickers, cyber criminals and swindlers of all stripes.
Long gone are the posters for Al Capone, "Machine Gun" Kelly, Bonnie and Clyde, and John Gotti. Osama bin Laden now tops the FBI's famous Most Wanted List. See a who's who of reputed wise guys »
Of all of the FBI's missions, the most pressing, Mueller said, is the need to stem the flow of biological, chemical and nuclear weapons to individuals and groups able and willing to use them.
"[We must] make certain that we do everything we possibly can to keep weapons of mass destruction out of the hands of terrorists, domestically and overseas as well, because Americans are all around the world now, and are targets -- and that's the principle concern," Mueller told CNN.
While supporters and critics debate the techniques used, there is general agreement the FBI has played an important role in keeping the nation safe since September 11, 2001.
Said critic Grassley, "We know they are fulfilling their mission when nothing happens to harm us, when we have another day, week and year free from a terrorist attack and violent crime."