As FBI Director Robert Mueller sat down with ABC News for a rare interview just as the agency was getting ready to celebrate its 100th anniversary, it became quite clear what drives Mueller and what now defines his agency.
"We lost 3,000 lives on Sept. 11. We shouldn't have lost one," Mueller said.
Asked what is his biggest concern right now, Mueller, the former Marine, was his blunt, as usual.
"I think we still have to be very concerned about individuals being trained in camps in the sanctuary between Pakistan and Afghanistan," Mueller said. "We cannot ... let down our guard at all because there are persons still being trained how to undertake attacks in Europe and the United States."
Mueller said that U.S. and international intelligence agencies have improved and are increasingly successful at disrupting plots. But, he said, the fact remains that al Qaeda is a metamorphic terror organization still on the attack.
"It shows that they understand the precautions and [are] trying to undertake new and innovative ways to attack us," Mueller said, as he began to lay out the different kinds of terror threats.
"But also the concern is homegrown groups ... where you have individuals who may be radicalized by one or more individuals, and then, encouraged through the Internet ... establish their own capabilities," he explained.
"And then you'll have a hybrid," he said." You'll have those who will not have traveled to Pakistan to gain the expertise but nonetheless ascribe to the radical extremist ideology and undertake their own attacks."
Mueller also warned, the threat of domestic terror remains quite real. "We cannot forget [Timothy] McVeigh and what occurred in Oklahoma City in 1995 ... the most devastating terrorist attack in United States history before Sept. 11 was the Oklahoma City bombing, which was undertaken by one of our very own."
And there have been significant victories, like the arrest of individuals last summer who allegedly sought to blow up pipelines at New York's John F. Kennedy International Airport and others who allegedly plotted to carry out an attack at Fort Dix in New Jersey.
Additionally, the plan of suspected al Qaeda operative Iyman Faris to destroy the Brooklyn Bridge ended with his 2003 guilty plea and a 20-year prison sentence. Several of his associates, who were based in Ohio, have also been imprisoned.
After the 9/11 attacks, the FBI and CIA faced criticism for losing the trail of several of the hijackers due to interagency infighting and restrictive guidelines that hampered law enforcement from sharing intelligence information.
In response to the criticism, Mueller has tried to transition the bureau by updating technology, hiring more translators and analysts, and making changes to agent training, all part of a strategy to try to transform the bureau to be both a law enforcement agency and an intelligence agency.
With the disclosure of the National Security Agency's warrantless wiretapping program, and questions about the agencies widespread use -- and abuse -- of national security letters, Mueller has had to walk the line between civil rights and security.
Mueller classified some of those missteps -- which occurred during his tenure and in previous decades -- as the darkest moments in the FBI's 100-year history.