Mark Coles profiles the long-serving and influential head of the FBI, Robert Mueller. Mueller took on the job one week before the 2001 World Trade Center attacks. Now, as he approaches retirement, the FBI is again dealing with the aftermath of an attack on American soil.
The squared-jawed Princeton graduate was decorated for bravery during the Vietnam War before training as a lawyer. Dissatisfied with private practice, he found a government job as assistant US attorney in San Francisco - a move which marked the beginning of a steady climb to the top of law enforcement in America.
He transformed the FBI from an organisation that solves crimes into one that also seeks to prevent terrorist attacks. He's now the longest-serving FBI director since J Edgar Hoover. Yet surprisingly little is known about him.