Diplomatic Immunity
Diplomatic Immunity is very important for genuine diplomats, it is in place to prevent vexatious accusations being made against a countries representatives.
The "tragic accident" caused by negligence is clearly not a vexatious accusation. The facts are well established, the evidence is likely to stand up in a court of law in both countries, and there is the principle that diplomatic immunity can be removed in cases where there is a genuine case.
If the US believe causing death isn't grounds for extradition then they can fuck right off for extradition for any crime less than death.
Biting the hand that feeds IT
