Mr Dunn, 19, died when his motorbike collided with a car near RAF Croughton in Northamptonshire in 2019.
Suspect Anne Sacoolas returned to the US, claiming diplomatic immunity. But now the Prime Minister Boris Johnson says he and President Joe Biden are “working together” on the case.
Home and Legal Affairs correspondent Dominic Casciani, and Washington reporter Hannah Long-Higgins explain the case and the complex world of diplomatic immunity.
Video journalist: Alex Dackevych
Additional filming by Roderick MaCleod