From the (US) Washington Post:
During the investigation, the American woman cooperated “fully” with police, and even assured them “she had no plans to leave the country in the near future,” Northamptonshire Police Superintendent Sarah Johnson said in a statement Saturday.
But then police learned the driver would be claiming diplomatic immunity, and they made an immediate waiver request to the U.S. Embassy in London, a spokeswoman told The Post. That request was denied, she said.
“We’re disgusted, appalled, how she could be having this cloak wrapped around her,” Dunn’s father, Tim Dunn, told Sky News. “I’m angry that someone could do this and then get on a plane and go.
“I can’t believe she’s living with herself,” he added.