By Daniel Sandford
BBC News, Washington
For Robert Mueller, it is personal.
Robert Mueller has issued a public letter regarding the release of Megrahi
The FBI director has mounted a scathing attack on the Scottish justice secretary over the release of convicted Lockerbie bomber Abdulbaset al-Megrahi.
He admits it is personal in a furious public letter, pointing out that in 1991 he was the assistant attorney general in charge of investigating Megrahi.
In its initial response the Scottish government has tried to make something of Mr Mueller's involvement, and pointed out that many relatives of those who died in the Lockerbie bombing had supported the compassionate release.
But to dismiss Mr Mueller's letter as the personal views of a man with strong ties to the case is to completely misunderstand how the release is being seen in the United States.
Two things have caused the most outrage.
Firstly, the relatives of those killed have closely observed the Libyan leader, Col Muammar Gaddafi, over the years.
They predicted that Megrahi would get a hero's welcome in Libya.
Abdelbaset al-Megrahi received a hero's welcome in Libya
They were dismissed, and it was only when the pictures were broadcast around the world that everyone realised the point they were making.
Secondly, they regard Mr MacAskill's defence of his actions, at his press conference, and live on many news programmes afterwards, as sanctimonious in the extreme.
"Who is he to show mercy to the man who took away the father of my children?" one mother asked me. "Who is he to preach about compassion? What compassion has he shown us?"
To the American families the Scottish government has appeared naive and hopelessly out of its depth.
The relatives have powerful friends in the Senate, and in the FBI - people who have been involved in this case going back to 1988, when Mr MacAskill was a minor politician campaigning against the poll tax.
The relatives successfully mobilised their friends against the decision, but it only proved how impotent the United States has been in influencing the outcome.
Mr MacAskill stuck to his guns and made the decision he felt was right.
But, to coin a phrase, hell hath no fury like an American administration ignored.
The president said the release was a "mistake", the attorney general said there was "no justification for it".
It took the man who knows many of the relatives personally, however, to put in writing the raw anger and incomprehension.
In a letter of six short paragraphs, Mr Mueller said he was "outraged" at an "inexplicable action" that is "detrimental to the cause of justice", "makes a mockery of the rule of law", and "gives comfort to terrorists".
He also said the decision makes "a mockery of the grief of the families", and was made "without regard to the views of your partners in the investigation".
Those are strong words from a director of the FBI in an official letter that he knows will be made public by his press office.