The final negotiated agreement said that the two men would be tried by a Scottish court, under Scottish law, in The Hague in the Netherlands. Distinct from the international court there, the three-judge Scottish court would ensure that the men faced justice under the laws of the country where their accused crime had been committed.

Allowing the Scots to move forward meant some concessions by the US. The big one was taking the death penalty, prohibited in Scotland, off the table. Mueller badly wanted the death penalty. Mueller, like many prosecutors and law enforcement officials, is a strong proponent of capital punishment, but he believes it should be reserved for only egregious crimes. “It has to be especially heinous, and you have to be 100 percent sure he’s guilty,” he says. This case met that criteria. “There’s never closure. If there can’t be closure, there should be justice—both for the victims as well as the society at large,” he says.

An old US military facility, Kamp Van Zeist, was converted to an elaborate jail and courtroom in The Hague, and the Dutch formally surrendered the two Libyans to Scottish police. The trial began in May 2000. For nine months, the court heard testimony from around the world. In what many observers saw as a political verdict, Al Megrahi was found guilty and Fhimah was found not guilty.

With barely 24 hours notice, Marquise and victim family members raced from the United States to be in the courtroom to hear the verdict. The morning of the verdict in 2001, Mueller was just days into his tenure as acting deputy US attorney general—filling in for the start of the George W. Bush administration in the department’s No. 2 role as attorney general John Ashcroft got himself situated.

That day, Mueller awoke early and joined with victims’ families and other officials in Washington, who watched the verdict announcement via a satellite hookup. To him, it was a chance for some closure—but the investigation would go on. As he told the media, “The United States remains vigilant in its pursuit to bring to justice any other individuals who may have been involved in the conspiracy to bring down Pan Am Flight 103.”

The Scotbom case would leave a deep imprint on Mueller; one of his first actions as FBI director was to recruit Kathryn Turman, who had served as the liaison to the Pan Am 103 victim families during the trial, to head the FBI’s Victim Services Division, helping to elevate the role and responsibility of the FBI in dealing with crime victims.

Just months after that 20th anniversary ceremony with Mueller at Arlington National Cemetery, in the summer of 2009, Scotland released a terminally ill Megrahi from prison after a lengthy appeals process, and sent him back to Libya. The decision was made, the Scottish minister of justice reported, on “compassionate grounds.” Few involved on the US side believed the terrorist deserved compassion. Megrahi was greeted as a hero on the tarmac in Libya—rose petals, cheering crowds. The US consensus remained that he should rot in prison.

The idea that Megrahi could walk out of prison on “compassionate” ground made a mockery of everything that Mueller had dedicated his life to fighting and doing. Amid a series of tepid official condemnations—President Obama labeled it “highly objectionable”—Mueller fired off a letter to Scottish minister Kenny MacAskill that stood out for its raw pain, anger, and deep sorrow.

“Over the years I have been a prosecutor, and recently as the Director of the FBI, I have made it a practice not to comment on the actions of other prosecutors, since only the prosecutor handling the case has all the facts and the law before him in reaching the appropriate decision,” Mueller began. “Your decision to release Megrahi causes me to abandon that practice in this case. I do so because I am familiar with the facts, and the law, having been the Assistant Attorney General in charge of the investigation and indictment of Megrahi in 1991. And I do so because I am outraged at your decision, blithely defended on the grounds of ‘compassion.’”