LONDON — The family of Harry Dunn, the teenage motorcyclist who was killed in a crash in Britain in August, is suing the Trump administration for “lawless misconduct,” a spokesman for the family said on Tuesday.

Mr. Dunn, 19, died after his motorcycle collided with a car that the police said had been traveling on the wrong side of the road on Aug. 27 in Brackley, a town about 60 miles northwest of London.

The case ignited a diplomatic tug-of-war between Britain and the United States after the woman thought to be driving the car, Anne Sacoolas, the wife of an American diplomat, claimed immunity and left the country a little over two weeks after the accident.

Seeking resolution in the case, Mr. Dunn’s parents have petitioned the highest echelons of government on both sides of the Atlantic, culminating with a meeting at the White House this month that left them disillusioned after President Trump unexpectedly announced that Ms. Sacoolas was in an adjacent room and wanted to meet with them. They declined to do so.