LONDON — The United States has rejected Britain’s request for the extradition of an American woman who fled the country after she was involved in an accident in which a teenager died last year. The State Department called the request “highly inappropriate,” deepening the diplomatic stalemate over the case.

The woman, Anne Sacoolas, who is the wife of a former American diplomat, was charged with causing death by dangerous driving in December after the Northamptonshire Police suspected her of traveling on the wrong side of the road.

The authorities said she was behind the wheel when her car collided with the motorcycle ridden by Harry Dunn, 19, in August in Brackley, a town about 60 miles northwest of London and close to a Royal Air Force base that hosts a United States Air Force communication station.

[Update: U.S. refuses extradition in fatal crash, prompting anger in the U.K.]

As the police sought to question Ms. Sacoolas, who was 42 at the time, she fled the country, spurring a diplomatic tug of war between Britain and the United States and enraging Mr. Dunn’s relatives. A campaign by his parents seeking her return to Britain has led them all the way to the White House.