LONDON — British Prime Minister Boris Johnson said Monday that he hoped a U.S. diplomat's wife involved in a fatal car crash in which a 19-year-old was killed would return to the United Kingdom, and that he would raise the issue with the White House if necessary.

Flowers at the site of a fatal crash in Northamptonshire, England. Reuters TV

"I do not think that it can be right to use the process of diplomatic immunity for this type of purpose,” Johnson said in a television interview.

The prime minister said that the U.K. was raising the issue with the U.S. ambassador Monday.

“If we can't resolve it, then of course I will be raising it myself personally with the White House," he added.

Harry Dunn, 19, died in the collision Aug. 27 in Croughton, a small village in central England, near an air force base that is used by the United States, Northamptonshire Police announced in late August.

Dunn was on his motorcycle when he collided with a Volvo SUV traveling in the opposite direction, police said.

Anne Sacoolas, 42, is being treated as a suspect in the investigation, police said.

It is not clear when exactly Sacoolas departed the U.K., but police told NBC News that they were advised Sept. 16 that the suspect had already left the country.

Harry Dunn, 19, was killed in August when his motorcycle collided with a Volvo. Courtesy of the Dunn Family

The head of the Northamptonshire Police, Nick Adderley, said in a tweet that he had written “in the strongest terms” to the U.S. Embassy, urging them to waive the woman’s immunity.

“We want to see justice done for the family,” Adderley told the BBC on Monday.

A U.S. Embassy spokesman said in a statement that immunity is rarely waived, but that cases like this “receive intense attention at senior levels and are considered carefully given the global impact such decisions carry.”

During the initial investigation, the police received assurances from U.S. authorities that Sacoolas would cooperate fully with police and wouldn’t be leaving the U.K., Adderley said.

“The next minute we found out that actually, she and her family had left, so it is really disappointing, which prompted the letter from me to say we want to see justice done and the family have a right to see justice done,” he said.