LONDON — The United States has formally turned down Britain’s extradition request for an American woman who was involved in a car accident that killed a teenager last year, a decision that the British government called “a denial of justice.”
The police have said that the woman, Anne Sacoolas, was driving a car on the wrong side of the road in August when it collided with a motorcycle ridden by Harry Dunn, 19. She fled Britain shortly afterward.
At the time of the accident, which occurred in Brackley, a town about 60 miles northwest of London, Ms. Sacoolas’s husband was working for the United States government at a British military base, and American officials assert that she had diplomatic immunity, shielding her from prosecution. But in December, British prosecutors charged her with causing death by dangerous driving.
The State Department said in a statement on Thursday that it had denied extradition, which it said “would render the invocation of diplomatic immunity a practical nullity and would set an extraordinarily troubling precedent.”
A spokesman for Mr. Dunn’s family, Radd Seiger, said on Friday that they had taken the news “in our stride” and would not give up, and he had harsh words for the Trump administration.
“The reality is that this administration, which we say is behaving lawlessly and taking a wrecking ball to one of the greatest alliances in the world, they won’t be around forever whereas that extradition request will be,” Mr. Seiger told BBC Radio 4.
“We are disappointed in this decision, which appears to be a denial of justice,” a spokeswoman for the Home Office, the British government department handling the matter, said on Friday. “We are urgently considering our options.”
Mr. Dunn’s parents have gone all the way to the White House to seek justice for their son, meeting with President Trump in October and refusing to meet Ms. Sacoolas, who Mr. Trump said was waiting in a nearby room.
Charlotte Charles, Mr. Dunn’s mother, said at the time that she and her husband would have loved to meet Ms. Sacoolas, but on their terms, and on British soil.
Prime Minister Boris Johnson said earlier this week that the chances that the United States would respond favorably to the request were very low. Andrea Leadsom, the British lawmaker in whose district Mr. Dunn’s parents live, is scheduled to meet with the United States ambassador, Woody Johnson, in London on Thursday, according to the BBC.
Dominic Raab, the British foreign secretary, said he had spoken with Mr. Johnson on Friday morning and told him that “the U.K. would have acted differently if this had been a U.K. diplomat serving in the U.S.”
Now, said Mr. Seiger, the spokesman for Mr. Dunn’s family, “we will simply plot and plan for a reasonable administration to come in one day and to reverse this decision.”