Mueller sent a scathing letter to Justice Secretary Kenny MacAskill, who cited compassionate grounds in his decision to let Abdel Baset al-Megrahi return to Libya because he has prostate cancer and was given only months to live by British doctors.
The angry tone of the letter is out of character with the normally reserved Mueller, indicating his outrage is personal as well as professional. He also sent copies to the families of the Lockerbie victims.
"I have made it a practice not to comment on the actions of other prosecutors," Mueller wrote. "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. ... And I do so because I am outraged at your decision, blithely defended on the grounds of 'compassion."'
Before he became FBI director, Mueller spent years as a Justice Department lawyer leading the investigation into the 1988 airplane bombing that killed 270 people, most of them Americans.
Mueller said Thursday's release was "as inexplicable as it is detrimental to the cause of justice. Indeed your action makes a mockery of the rule of law."
His letter was dated Friday, and was made public Saturday.
Releasing the convicted bomber "gives comfort to terrorists around the world who now believe that regardless of the quality of the investigation ... the terrorist will be freed by one man's exercise of 'compassion."'
A statement from Scotland's government on Saturday noted Mueller has "strong views" because of his involvement in the case. "But he should also be aware that while many families have opposed Mr. MacAskill's decision, many others have supported it," the statement said.
Bert Ammerman of Riverside, N.J., who lost his brother Tom Ammerman in the bombing, praised Mueller for the "frankness and honesty" in his condemnation of the release.
Mueller recounted his own emotional experiences leading the investigation -- seeing a teenage victim's single sneaker, a Syracuse University sweatshirt, toys in the suitcase of a businessman heading home to see his wife and children for Christmas.
"Your action," he wrote MacAskill, "makes a mockery of the grief of the families who lost their own on December 21, 1988. You could not have spent much time with the families, certainly not as much time as others involved in the investigation and prosecution."
He ended the Lockerbie letter with a frustrated question: "Where, I ask, is the justice?"
President Barack Obama on Friday called the elaborate homecoming in Libya for the freed bomber "highly objectionable."?"
President Obama on Friday called the elaborate homecoming in Libya for the freed bomber "highly objectionable."