The United States has declined Britain's request for the extradition of Anne Sacoolas - who was involved in a ca crash that killed a 19-year-old British teenager last year.
The crash happened in August and, at the time he was killed, Harry Dunn was riding his motorbike.
In a statement, the State Department said Sacoolas extradition would set a troubling precedent.
"At the time the accident occurred, and for the duration of her stay in the UK, the U.S. citizen driver in this case had immunity from criminal jurisdiction."
"If the United States were to grant the UK's extradition request, it would render the invocation of diplomatic immunity a practical nullity and would set an extraordinarily troubling precedent," the statement said.
Either way, British Prime Minister Boris Johnson decried Sacoolas' use of diplomatic immunity to leave Britain and has urged the US to reconsoder its position on the matter.