LONDON (AP) — Britain has sparked a diplomatic standoff with Washington by requesting the extradition of a U.S. woman charged over a road crash that killed a U.K. teenager.

The British government said late Friday it had formally asked for Anne Sacoolas to be sent to the U.K. to face a charge of causing death by dangerous driving. The U.S. State Department called the request “highly inappropriate” because it says Sacoolas had diplomatic immunity at the time of the crash.

Harry Dunn, 19, died in August after his motorbike collided with a car driven by Sacoolas outside RAF Croughton, a British military base in central England used by U.S. forces. Sacoolas, whose husband was an intelligence officer at the base, claimed diplomatic immunity and returned to the U.S. soon afterwards.

Under the Vienna Convention on Diplomatic Relations, families of diplomats are granted immunity from arrest or detention. British prosecutors, however, maintain that immunity does not apply to dependants of consular officials based outside London.

Dunn’s family has urged Sacoolas to return and face British justice, and met with U.S. President Donald Trump in Washington as part of their campaign.

Despite the cool U.S. response, the Dunn family said the extradition request was a “huge step towards achieving justice for Harry.”