LONDON — The mother of a British teenager who was killed in a crash in August has appealed to President Trump for help after the main suspect in the investigation, an American citizen, left Britain despite telling the police that she had no plans to do so.
Harry Dunn, 19, was killed after his motorcycle collided with a car traveling in the opposite direction in Brackley, a town in the East Midlands, on Aug. 27, near R.A.F. Croughton, a Royal Air Force base that is the site of a United States Air Force communication station.
The police said their investigation into the crash was complicated when the 42-year-old American woman who is the main suspect left the country.
The American was driving on the wrong side of the road when the crash occurred, the police said, and she is the wife of an American diplomat, and thus has diplomatic immunity, according to local news reports.
“Harry Dunn’s family deserves justice, and in order to achieve this, a full and thorough investigation, with the assistance of all parties involved, needs to take place,” Superintendent Sarah Johnson, head of operations for the Northamptonshire police, said in a statement on Saturday. She added that the police had sought documentation “to allow for the arrest and formal interview of the subject.”
The police are also working closely with the Foreign and Commonwealth Office, Ms. Johnson said, adding that they are “exploring all opportunities through diplomatic channels” to ensure the investigation’s progress.
Harry Dunn’s mother, Charlotte Charles, has been pushing the efforts to have the American woman’s immunity waived and have her come back to Britain.
“President Trump, please listen,” Ms. Charles told Sky News, the outlet that first reported on the identity of the 42-year-old woman. “We’re a family in ruin. We’re broken.”
“We can’t grieve,” she said, adding of the woman, “Please, please let her get back on a plane.”
The developments are likely to further strain the so-called special relationship, which has already been tested numerous times during the presidency of Donald Trump.
Mr. Trump has engaged in public spats with Sadiq Khan, the mayor of London and endorsed Boris Johnson as a strong candidate for prime minister while his predecessor, Theresa May, was still in office. Kim Darroch, the British ambassador to the United States, resigned after leaked cables said Mr. Trump was “radiating insecurity” and called his administration diplomatically “clumsy and inept.”
Still, Britain must walk a fine line, most notably because it is hoping to sign a trade deal with the United States after it leaves the European Union, a fraught process that is complicated by the fact that the Americans may make demands the British find unacceptable.
Though the Foreign Office did not formally confirm the identity of the American woman, Dominic Raab, the foreign secretary, offered his condolences to the victim’s family and released a statement addressing the matter.
Britain transmitted a formal request for a waiver of immunity to the American Embassy in London on Sept. 5. The embassy declined the waiver eight days later.
“I have called the U.S. ambassador to express the U.K.’s disappointment with their decision, and to urge the embassy to reconsider it,” Mr. Raab said. The American Embassy could not be reached for comment.
After news of the woman’s flight from Britain emerged, the news media, the British government and social media users stepped up pressure on the American government to reconsider her immunity and allow the police to resume questioning her in Britain.
“We have to get proper justice for Harry and closure for his family,” Andrea Leadsom, the secretary of state for business, energy and industrial strategy, wrote on Twitter on Saturday. A day earlier, she had met Harry Dunn’s family, who, she wrote, is “heartbroken.”
“Harry & his family have been wronged,” Angela Rayner, a member of Parliament for the opposition Labour Party, said in a tweet.
The victim’s parents said they would continue fighting to get justice for their son’s death.
“We are not going to be swept under the carpet,” Ms. Charles told ITV, a British news channel. “If that becomes his legacy, then we are going to carry on fighting, we’re not going to give up, we’re not going to go away,” she said.
“We can’t let our son die and nothing be answered for,” Tim Dunn, his father, said.