UK says immunity is 'no longer relevant' for American diplomat's wife involved in fatal crash British Foreign Secretary Dominic Raab says immunity is "no longer relevant."

LONDON -- An American diplomat's wife who was involved in a crash that killed a teenager in the United Kingdom is no longer covered by diplomatic immunity, according to British Foreign Secretary Dominic Raab.

In a recent letter to the parents of 19-year-old Harry Dunn, obtained by ABC News, Raab explained that immunity "is no longer relevant" because the diplomat's wife, 42-year-old Anne Sacoolas, has returned home to the United States. Raab said he and his staff have been "in constant discussions" with the U.S. government and embassy officials since the deadly traffic accident occurred over the summer.

"We have pressed strongly for a waiver of immunity, so that justice can be done in Harry's case," the British foreign secretary wrote. "Whilst the U.S. government has steadfastly declined to give that waiver, that is not the end of the matter."

"The question remains when such immunity comes to an end, regardless of any waiver. We have looked at this very carefully, as I wanted to be confident in the position before conveying it to you," Raab continued. "The U.K. government's position is that immunity, and therefore any question of waiver, is no longer relevant in Mrs. Sacoolas' case, because she has returned home. The U.S. have now informed us that they too consider that immunity is no longer pertinent."

Dunn was riding his motorcycle along a roadway in the village of Croughton, England, on the night of Aug. 27, when a car traveling in the opposite direction on the wrong side of the road hit him head-on. The teen was taken to a hospital in the nearby city of Oxford, where he died soon after, according to Northamptonshire Police.

The crash occurred less than a mile down the road from Royal Air Force Croughton, commonly known as RAF Croughton, which is a British military station that houses an intelligence-gathering base operated by the United States Air Force. Sacoolas, whose husband is a U.S. diplomat assigned to the United Kingdom, is believed to have been the one driving the car.

Northamptonshire Police are treating Sacoolas as a suspect in the fatal crash investigation, although they have not officially named her.

Sacoolas fled the United Kingdom after apparently claiming diplomatic immunity, which protects diplomats and their family members from prosecution or lawsuits under the host nation's laws.

Northamptonshire Police superintendent Sarah Johnson said the woman left the country after allegedly telling investigators she had no plans to do so in the near future. Police intended to arrest and formally interview her, and they are "now exploring all opportunities through diplomatic channels to ensure that the investigation continues to progress," according to Johnson.

“Our investigation into the death of Harry Dunn continues at pace," Johnson said in a statement Sunday. "Northamptonshire Police remains absolutely committed to getting Harry and his family justice and we are doing everything on our side to ensure that a full and thorough investigation, with the assistance of all parties involved, takes place, in order for this to be achieved."

Sacoolas' lawyer, Amy Jeffress, said her client has "fully cooperated" with police and wants to meet with Dunn's parents to apologize.

"Anne is devastated by this tragic accident," Jeffress told ABC News in a statement Saturday. "No loss compares to the death of a child and Anne extends her deepest sympathy to Harry Dunn’s family."

"The media reporting has been inaccurate in many respects," she continued. "To begin with, Anne fully cooperated with the police and the investigation. She spoke with authorities at the scene of the accident and met with the Northampton police at her home the following day. She will continue to cooperate with the investigation. Anne would like to meet with Mr. Dunn’s parents so that she can express her deepest sympathies and apologies for this tragic accident. We have been in contact with the family’s attorneys and look forward to hearing from them."

In a sit-down interview with ABC News in London last week, Dunn's parents said they were "shocked" to learn that Sacoolas had left the country after the accident.

"It's hard to understand, for me personally, how anybody could -- even though being an accident -- actually kill somebody, but then up and leave and just walk away from it," said Dunn's father, Tim Dunn.

The family has urged Sacoolas to return.

"Just face up to what you've done," Dunn's mother, Charlotte Charles, said tearfully. "I can't see how you can move on with your family and your life, without facing the family that you've destroyed."

Dunn's parents have traveled to the United States this week to continue seeking justice for their late son. They are meeting with journalists and politicians in New York City and Washington, D.C. "as they reach out for support from all Americans and to ask them to put pressure on the U.S. administration to do the right thing," according to a press release from a family spokesperson.

"Harry's family will simply not leave matters where they are and will do whatever it takes," the spokesperson said, "including taking legal action if necessary."

ABC News' Katie Bosland, Rashid Haddou and Bruno Roeber contributed to this report.