Anne Sacoolas, 42, who Johnson named as the suspect in the investigation on Monday, left the UK despite assuring the police she would remain the country in the near future, police said.
The incident took place in the village of Croughton, which is home to a Royal Air Force station controlled by the US Air Force.
“I do not think that it can be right to use the process of diplomatic immunity for this type of purpose,” Johnson said on Monday. “I hope that Anne Sacoolas will come back and engage properly with the processes of law as they’re carried out in this country.”
Johnson said he would going to raise the issue with US Ambassador to the UK, Woody Johnson, adding that “if we can’t resolve it then of course I will be raising it myself personally with the White House.”
“We are delighted to hear that the Prime Minister has committed to help us, and has named the suspect, Anne Sacoolas,” Dunn’s family said in a statement to CNN.
“We remain laser-focused on having her return to the UK and we will not rest until that happens. We urge Mr. Johnson to do whatever it takes to bring Anne back to face justice.”
CNN’s attempts to contact Sacoolas were unsuccessful.
‘Immunity rarely waived’
Under the 1961 Vienna Convention, diplomats and their family members are typically immune from prosecution in their host country.
The US State Department told CNN Saturday that diplomatic immunity is “rarely waived.”
“Any questions regarding a waiver of immunity with regard to our diplomats and their family members overseas in a case like this receive intense attention at senior levels and are considered carefully given the global impact such decisions carry,” it added.
The US Embassy in London refused to comment on the identity of the suspect for privacy and security reasons.