LONDON (AP) — The parents of a British teen who was killed in a crash lost a court battle with the U.K. government Tuesday over whether an American woman involved in the collision had diplomatic immunity.

The family has been seeking justice for 19-year-old Harry Dunn, who died after his motorbike crashed into a car driven on the wrong side of the road outside a U.S. airbase in central England last August.

The car’s driver, Anne Sacoolas, left for the U.S. several weeks after the collision. Officials said she was entitled to diplomatic immunity because her husband worked at the airbase.

Sacoolas, 43, was charged in December with causing death by dangerous driving, but the U.S. State Department rejected a request to extradite her to Britain to face trial.

Dunn’s parents, Charlotte Charles and Tim Dunn, launched the court case to argue that Britain’s Foreign Office wrongly decided Sacoolas had diplomatic immunity and unlawfully obstructed the police investigation into their son’s death. Their lawyer said Sacoolas had “no duties at all” at the base.

But two judges rejected that Tuesday, ruling that the American had diplomatic immunity “on arrival in the U.K.” under the Vienna Convention on Diplomatic Relations, and that she “enjoyed immunity from U.K. criminal jurisdiction at the time of Harry’s death.”

The teen’s mother said she was determined to continue finding justice for her son. A family spokesman said they would appeal the ruling.

“I promised my boy I would get him justice and that is just what we are going to do. No one is going to stand in our way," she said after the ruling.

She was backed by British Foreign Secretary Dominic Raab, who said he stands with the family.

“We’re clear that Anne Sacoolas needs to face justice in the U.K, and we will support the family with their legal claim in the U.S.,” Raab said.