LONDON - British Foreign Secretary Dominic Raab on Friday condemned the U.S. government for its refusal to extradite an American woman charged in the death of the British teenager Harry Dunn.

The British government "would have acted differently if this had been a U.K. diplomat serving in the U.S.," Raab said.

On Thursday, the U.S. government denied a British extradition request pertaining to Anne Sacoolas, an American diplomat's wife. Sacoolas admitted to driving on the wrong side of the road when she collided with 19-year-old Dunn in August, but she claimed immunity and fled to the United States. She has been formally charged with "causing death by dangerous driving."

The high-profile case has prompted interventions by both President Donald Trump and British Prime Minister Boris Johnson.

A State Department spokesman on Thursday expressed condolences to Dunn's family but told The Washington Post in a statement: "At the time the accident occurred, and for the duration of her stay in the U.K, the U.S. citizen driver in this case had immunity from criminal jurisdiction. If the United States were to grant the UK's extradition request, it would render the invocation of diplomatic immunity a practical nullity and would set an extraordinarily troubling precedent."

Raab called the U.S. decision a "denial of justice." He said he had expressed "the government's disappointment" to the U.S. ambassador in London and that the British government was "urgently considering our options."

Dunn's mother, Charlotte Charles, told Sky News on Friday, that "it is a blow, but it's one that we expected." She added that they would continue pushing to get Sacoolas back to Britain.

Harry's father, Tim Dunn, told the broadcaster, "at the end of the day, this lady has killed our son, by an accident, but she has to answer for that."

A spokesman for the family called the decision "one of the darkest days in the history of this special relationship" between the two allies.

The U.S. government "is effectively saying it's OK for American service personnel to come to the U.K., kill our children and get on the next plane home," the spokesman, Radd Seiger, told The Post Friday. "And I can assure your leaders that is not what's going to happen. This is far from over."

At the time of the collision, Sacoolas was living with her husband near the Royal Air Force Croughton station, which is operated by the U.S. Air Force.

Under the 1961 Vienna Convention on Diplomatic Relations, family members of diplomats are covered by immunity while living abroad. But countries can also decide to waive immunity to allow a diplomat, or family member, to face serious charges.

Nick Vamos, a former head of extradition at Britain's Crown Prosecution Service, said the Vienna Convention sets out the "high level rules" but is not "a complete and comprehensive rule book." He said the law of diplomatic immunity appears to have been interpreted differently in Britain and in the United States, adding "there is no court in which that dispute can be resolved."

The U.K.'s position, he said, was that Sacoolas had immunity, but then lost it when she left the U.K. He said the United States' position, though not spelled out by the State Department, could be "that if she couldn't be prosecuted in the U.K. when she was here, then she shouldn't be obliged to return through legal process. It's a gray area."

Given that the U.S. and the U.K. are close allies, he said, "it's politically inconceivable for the U.K. to bring legal proceedings against [Secretary of State] Mike Pompeo exercising his discretion in this case." He said that Sacoolas could be extradited via another country if she leaves the United States, but "if she stays the rest of her life in the U.S. and there's no change of heart by the U.S. administration, this brings it to the end."

Sacoolas's refusal to return has caused an uproar in Britain. And Dunn's parents have mounted a vigorous campaign, appealing directly to Trump and meeting with him at the White House in October.

In a surprise move, the U.S. president told them Sacoolas was in the building and ready to meet with them. He wanted everyone to have a "hug and makeup moment," a person with knowledge of the discussions told The Post at the time.

Dunn's parents declined to meet with Sacoolas outside of formal legal proceedings. Seiger, the spokesman, described the episode as an "ambush."

On Thursday, Seiger said the family plans to discuss possible next steps with British government officials, including floating the idea of shutting down RAF bases operated by the U.S. military within Britain. He said if nothing works under Trump, the family plans to take the fight to a new U.S. administration in the future.

"These extradition requests never go away," he said. "This will hang over Anne Sacoolas's head forever, and one day, a reasonable administration will come in, will look at this and say, no, this is not how we treat our strongest ally. She has killed a young man, and she has to go back."

Under British law, causing death by dangerous driving is punishable by up to 14 years in prison.

An attorney for Sacoolas, Amy Jeffress, declined to comment on the latest development. But Jeffress has previously said a 14-year sentence for "a terrible but unintentional accident" would be a disproportionate punishment.

"Anne is devastated by this tragic accident and would do anything she could to bring Harry back," Jeffress said in a Jan. 10 statement provided to The Post. "She continues to grieve for Harry and his family.… We remain willing to work with the UK authorities to identify a path forward."

Andrea Leadsom, Britain's business secretary and the lawmaker representing the Dunn family's constituency, met with the U.S. ambassador on to discuss the case.

"This was a tragic road accident where a much loved young man died. His family are heartbroken," she tweeted.

She added: "The person who has been charged by the CPS should come back to the U.K. #Justiceforharry."

This article was written by Meagan Flynn and Karla Adam, reporters for The Washington Post.