Robert S. Mueller arrived at the FBI on September 4, 2001, one week before the terror attacks that shook the nation and the world. Twelve years later, the longest-serving Director since J. Edgar Hoover is widely credited with transforming the organization while maintaining the bedrock ideals that have been the FBI’s hallmark for more than a century: fidelity, bravery, and integrity.

“When I first came on board, I thought I had a fair idea of what to expect,” Mueller said recently during a farewell ceremony at FBI Headquarters to honor his tenure as Director. “But the September 11 attacks altered every expectation.”

Mueller, who will step down as Director next week, reshaped the Bureau from a traditional law enforcement agency to a threat-focused, intelligence-based national security organization, said FBI Deputy Director Sean Joyce. “He guided the FBI through a period of tremendous change.”

Director Mueller receives an ovation from President Obama and his appointee for FBI Director James Comey during a White House ceremony in June

At the ceremony attended by employees, incoming Director James Comey, and former FBI Directors William Webster, William Sessions, and Louis Freeh, Joyce noted that Mueller has served the American people for four decades—as a decorated Marine, a prosecutor and former U.S. attorney, and leader of the FBI.

“As a prosecutor,” Mueller said, “I had worked closely with the FBI over the years. But I had not witnessed firsthand the FBI’s incredible response in times of crisis or the ability of the Bureau’s men and women to band together to do what is needed—without fanfare or drama—and with a level of efficiency and excellence that is unrivaled.”

As FBI Director for 12 years—he agreed to extend his 10-year term at the request of President Obama—Mueller faced the daunting task of keeping the country safe from terror attacks while maintaining the Bureau’s established crime-fighting role. “People talk about change in the Bureau,” he said, “but I consider what we were doing as augmenting the traditional capabilities the FBI already possessed.”

Mueller said he still worries about the threat of terrorism—international and domestic—and cyber crime is a growing danger to our national security. “Should a terrorist utilize cyber capabilities to undertake an attack, it could be devastating,” he said. “We have to be prepared.”

The American people expect the FBI to be vigilant against such attacks while continuing to put corrupt politicians in jail, arrest violent criminals and scam artists, and keep the country’s secrets safe from spies. “Those are substantial responsibilities, and we are up to the task,” Mueller said, “but we need the support and the funding to exercise our capabilities.”

During his tenure as Director, Mueller regularly spoke of the “FBI family” and the Bureau’s responsibility to help keep the nation safe. “We have unique mission and a unique legacy that has been passed down to us,” he said. “People in the FBI are tremendously proud to be a part of that legacy.” For more than a century, he added, “the FBI has stood for the best of America. And we have done this by adhering to our motto of fidelity, bravery, and integrity. … It has been my greatest honor and privilege to have been part of the FBI family for the past 12 years.”