Diplomatic immunity
A spokesperson for Dunn's parents said the decision was 'further compounding the family's misery'.
The teen's father, Tim, said: "They made a mistake, someone has made a mistake."
Meanwhile, the home secretary played down suggestions Sacoolas could be extradited from the United States.
Suspect Anne Sacoolas said she had been 'devastated' by the 'tragic accident'.
The teenager's parents are travelling to the US in a bid to pressure the administration into doing 'the right thing'.
The wife of a US diplomat is suspected of involvement in the teenager's death, and the case has sparked a massive international row.
Dunn was killed when his motorbike crashed into a car outside an RAF base on August 27.
The development comes as Harry Dunn's mother said she is "disgusted" at how the case is unfolding.
US president said Americans driving on the other side of the road in the UK “happens”.
Prime minister vows to raise situation "personally" with White House if issue cannot be resolved.