LONDON — British police officers investigating a crash that killed a teenage motorcyclist in Britain in August will travel to the United States to interview the American suspect who claimed diplomatic immunity and fled the country shortly after the accident, the police said on Tuesday.
The development is the latest in a weekslong episode that has engaged Britain and the United States in a diplomatic push-and-pull as the teenager’s family traveled between the two countries in a search for answers and justice.
The suspect in the case, Anne Sacoolas, 42, is the wife of an American diplomat who worked at a Royal Air Force base that hosts a United States Air Force communication station. As such, she was entitled to immunity under a 1995 treaty between Britain and the United States, Britain’s foreign secretary, Dominic Raab, told Parliament on Monday.
She is suspected of being the driver of a car that the police say was traveling on the wrong side of the road when it collided with a motorcycle ridden by Harry Dunn, 19, on Aug. 27 in Brackley, a town about 60 miles northwest of London.
Although Ms. Sacoolas cooperated with the local police immediately after the crash, the next day she informed them of her diplomatic immunity, Chief Constable Nick Adderley of Northamptonshire Police told reporters on Tuesday.
This effectively halted their communication with Ms. Sacoolas.
Mr. Adderley said on Tuesday that Ms. Sacoolas had agreed to be interviewed in the United States, and that the file of evidence the police submitted to the Crown Prosecution Service was incomplete without an account from her about the crash.
He said British officers would travel to the United States once their visas were approved.
British and American officials have said that Ms. Sacoolas’s claim of immunity is no longer relevant since she has returned home. A spokesman for the Crown Prosecution Service said in an email on Tuesday that it would decide its course of action once all the evidence from the police investigation had been gathered.
A spokesman for Harry Dunn’s family, Radd Seiger, said in a telephone interview on Tuesday that the family had not been told of Ms. Sacoolas’s agreeing to a police interview in the case, which has led to diplomatic tangling on both sides of the Atlantic.
The day after the crash, the United States Embassy in London told the Foreign Office that the wife of an air force base employee had been involved in a traffic accident, Mr. Raab told the House of Commons on Monday.
On the same day, the police applied to waive Ms. Sacoolas’s immunity in the case, Mr. Adderley said.
And despite an assertion by Washington that she was covered by immunity, Britain made a formal request to the United States Embassy on Sept. 5 to waive that protection.
That request was declined, and on Sept. 13 the embassy informed the British authorities that Ms. Sacoolas “would be leaving the country imminently, unless the U.K. had strong objections,” Mr. Raab said on Monday — the first public acknowledgment that the British government had been warned of Ms. Sacoolas’s impending departure.
Mr. Raab said that the government had “duly and immediately objected in clear and strong terms.” Even so, Ms. Sacoolas left Britain on Sept. 15. According to Mr. Raab, the Foreign Office was informed of her exit the next day.
Officials notified the police, Mr. Raab said, but asked them not to tell Mr. Dunn’s family for a day or two in order to give the government time to decide on a course of action.
The family was officially told 11 days after Ms. Sacoolas’s departure that the suspect in their son’s death had left the country, Mr. Raab said — though Mr. Adderley, the chief constable, said that the family had learned of her exit a week after the police found out. The chief constable also said that the police had delayed informing the family until after Mr. Dunn’s funeral.
Commenting on Mr. Raab’s statement to British lawmakers on Monday, Mr. Seiger, the family spokesman, said that they were disappointed.
“He could have told us a lot of that and he chose not to,” Mr. Seiger said. “The family simply struggle to believe anything he says.”
The teenager’s parents, Tim Dunn and Charlotte Charles, met with Mr. Raab two weeks ago, an exchange that Ms. Charles said had felt in part like “a publicity stunt.”
The couple were also disillusioned by a meeting they had last week with President Trump in the White House, having traveled to the United States after making a public appeal to the American authorities.
Shortly after Mr. Dunn and Ms Charles met the president, he informed them that Ms. Sacoolas was in a nearby room and wanted to meet with them. “I offered to bring the person in question in,” Mr. Trump told reporters last week. “They weren’t ready for it. But I did offer.”
“The bombshell was dropped not soon after we walked in the room,” Ms. Charles told reporters after the encounter. “We would still love to meet with her, but it has to be on our terms and on U.K. soil.”
A lawyer for Mr. Dunn’s family said afterward that Robert C. O’Brien, Mr. Trump’s national security adviser, who was present in the White House meeting, had shut down any possibility that Ms. Sacoolas would return to Britain.
While the president “left an olive branch on the table for a political solution,” said the lawyer, Mark Stephens, Mr. O’Brien said that Ms. Sacoolas was “not going back under any circumstances.”