LONDON — The diplomatic immunity of Anne Sacoolas, the American woman who is a suspect in a road crash that killed Harry Dunn, a British teenager, is “no longer relevant” in the case, Britain’s foreign secretary has said.
Ms. Sacoolas, the 42-year-old wife of a diplomat, fled Britain using diplomatic immunity after the police in Northamptonshire, England, suspected her of being the driver of a car that they say had been traveling on the wrong side of the road when it collided with a motorcycle ridden by Mr. Dunn, 19, in August.
Britain and the United States have been involved in a diplomatic tug-of-war over the case, but Dominic Raab, the British foreign secretary, said in a letter to the Dunn family on Saturday that Ms. Sacoolas’s immunity no longer applied because she had gone back to the United States.
“The U.K. Government’s position is that immunity, and therefore any question of waiver, is no longer relevant in Mrs. Sacoolas’s case, because she has returned home,” Mr. Raab said in the letter that was shared with The New York Times on Sunday.
“The U.S. have now informed us that they, too, consider that immunity is no longer pertinent,” Mr. Raab said.
He added that the matter was now in the hands of the Northamptonshire Police and the Crown Prosecution Service.
The news came as a spokesman for the Dunn family, Radd Seiger, said Ms. Sacoolas’s lawyer had also been in touch and said she wanted to meet.
“She reached out to me yesterday and we’ve agreed that she and I will get together at the earliest possible convenience,” Mr. Seiger said by phone on Sunday.
Ms. Sacoolas was said to be “devastated by this tragic incident,” according to a statement made on her behalf by her lawyer, Amy Jeffress. “No loss compares to the death of a child and Anne extends her deepest sympathy to Harry Dunn’s family,” the statement said.
Ms. Jeffress was not immediately available for comment on Sunday.
Charlotte Charles, Harry Dunn’s mother, told Sky News on Sunday that the apology was “the start of some closure” for the family.
“Having said that, as it’s nearly seven weeks now since we lost our boy, sorry just doesn’t cut it,” said Ms. Charles, who along with Tim Dunn, Harry Dunn’s father, was traveling to the United States on Sunday.
Commenting on Mr. Raab’s letter, Mr. Dunn said that “obviously we were right from the start.” Harry Dunn’s parents said this month that they would continue fighting to get justice in the case.
The crash occurred on Aug. 27 in Brackley, a town in Northamptonshire about 60 miles northwest of London and near a Royal Air Force base that hosts a United States Air Force communication station.
After the accident, Ms. Sacoolas told officers that she had no plans to travel, the police said, but her sudden departure prompted a diplomatic push-and-pull.
Britain made a formal request for a waiver of diplomatic immunity to the United States Embassy in London on Sept. 5. It was declined eight days later.
Saoirse Townshend, a London-based lawyer who specializes on extraditions, said in an email on Sunday that Britain could seek Ms. Sacoolas’s extradition, but in order to do so, the Crown Prosecution Service would have to formally charge Ms. Sacoolas with a crime first.
Nick Vamos, another extradition lawyer, said by email that “it’s not clear whether a U.S. court would extradite in these circumstances.”
“However,” he added, “Mrs. Sacoolas says she will voluntarily cooperate with the U.K. investigation, so extradition is unlikely to be needed.”
Prime Minister Boris Johnson of Britain said that he would appeal directly to President Trump for Ms. Sacoolas’s return, and he raised the case in a phone call with Mr. Trump on Wednesday, according to a statement from the prime minister’s office.
Mr. Trump expressed his condolences to Mr. Dunn’s family and “agreed to work together to find a way forward.”
Speaking to reporters on Wednesday, Mr. Trump described the death as “a terrible accident,” adding that it was “a very, very complex issue because we’re talking about diplomatic immunity.”
But he also expressed understanding for Ms. Sacoolas, suggesting that he had himself driven on the wrong side of the road in Britain.
“You know those are the opposite roads; that happens,” he said. “I won’t say it ever happened to me, but it did.”
Priti Patel, the British home secretary, was criticized on social media on Sunday after she avoided answering whether Britain would push for Ms. Sacoolas’s extradition.
Speaking to Andrew Marr on the BBC, Ms. Patel said that “this is an incredibly tragic case.” She added that, “the fact of the matter is right now it very much seems that the lady in question wants to start cooperating with the discussions and obviously the investigations, and I think we should support that.”