London — A deadly car accident in England has led to a diplomatic standoff with the United States. Harry Dunn, 19, was struck and killed on his motorcycle in late August by a vehicle allegedly driving on the wrong side of the road, and police say they were about to arrest a U.S. diplomat's wife in the case.

That American woman, a U.S. diplomat's wife, was allowed to leave Britain under diplomatic immunity, however.

CBS News correspondent Imtiaz Tyab said that has left Charlotte Charles and Tim Dunn in agony. Not only have they lost their son, they say, but a legal loophole for diplomats and their families has also robbed them of their ability to grieve.

"She killed our boy," Charles said in an emotional interview. "If it wasn't for her being under this supposed diplomatic immunity, it would have been a clear cut case… and it's just all turned into an absolute nightmare for us."

Harry Dunn is seen in a family photo.

Their nightmare started on a country road just outside Royal Air Force Base Croughton, which is used also as a U.S. intelligence and communications base.

The family said Anne Sacoolas, the diplomat's American wife, was driving on the wrong side of the road when she crashed into Dunn.

Police are treating her as a suspect, but days after the crash she flew back to the U.S. under diplomatic immunity.

"We've just been left to just accept that she's got this diplomatic immunity, and (the case is) swept under the carpet," Charles said.

Law professor David Glazier told CBS News that, right or wrong, it's a complicated case.

"The reason is that diplomatic immunity has been agreed to by states in order to protect diplomats in foreign countries, particularly those countries that might have more corrupt or less scrupulous legal systems," Glazier said.

But Prime Minister Boris Johnson urged the U.S. Embassy on Monday to re-think its decision to invoke immunity for Sacoolas.

"I do not think that it can be right to use the process of diplomatic immunity for this type of purpose," he said on a U.K. breakfast news show. "I hope that Anne Sacoolas will come back and will engage properly with the processes of law as they are carried out in this country. That's a point that we've raised or are raising today with the American Ambassador here in the U.K., and I hope it will be resolved very shortly. And to anticipate a question you might want to raise, if we can't resolve it then of course I will be raising it myself personally with the White House."

But that will be small comfort for two grieving parents.

Charles said they want Sacoolas to return to Britain to face justice, and "we want some acknowledgment from her that she's remorseful."

In a statement provided Monday morning to CBS News, the U.S. State Department expressed sympathy and extended condolences to the bereaved family, but said "questions regarding a waiver of immunity with regard to our diplomats and their family members overseas in a case like this receive intense attention at senior levels and are considered carefully given the global impact such decisions carry; immunity is rarely waived."

The State Department said it could not "speculate on what actions the British Government may take" and would not comment on "private diplomatic conversation with the British Government."