The family of Harry Dunn has said a meeting with Foreign Secretary Dominic Raab over the death of their son felt like a "publicity stunt" – as they confirmed they were launching a civil case against the suspect in the case.
Mr Dunn, 19, was killed when his motorbike crashed into a car on August 27.
The suspect, 42-year-old Anne Sacoolas, reportedly married to a US intelligence official, was granted diplomatic immunity following the crash.
The car was thought to have been driving on the wrong side of the road after leaving RAF Croughton in Northamptonshire – a military base used by the US Air Force.
Mr Raab met Harry's mother, Charlotte Charles, and father, Tim Dunn, on Wednesday afternoon after having talks with US Ambassador Woody Johnson on Tuesday.
Speaking after the meeting, Harry's mother told reporters she felt "let down by both governments".
She said: "I can't really see the point as to why we were invited to see Dominic Raab. We are no further forward than where we were this time last week.
"Part of me is feeling like it was just a publicity stunt on the UK government side to show they are trying to help.
"But, although he is engaging with us, we have no answers. We are really frustrated that we could spend half an hour or more with him and just come out with nothing."
The lawyer for the family said they were engaging lawyers to take a civil case against Mrs Sacoolas in America.
"Our position is that she doesn't have immunity and that waivers are always granted in these circumstances," Radd Seiger told reporters in Westminster.
"Now we can disclose to you we have brought lawyers on board ... We are going to Washington soon to help us get that justice for Harry."
Prime Minister Boris Johnson, Mr Raab and Northamptonshire Police have asked the US to consider waiving the immunity.
It has since emerged that Mrs Sacoolas was previously handed a fine for "failure to pay full time and attention" while driving in the state of Virginia in 2006.
Prior to meeting the US Ambassador, Mr Raab raised the case in a telephone call with US Secretary of State Mike Pompeo.
After Tuesday's meeting, a Foreign Office spokeswoman said: "The Foreign Secretary met the US Ambassador today and urged the US to reconsider its position and do the right thing by Harry Dunn's family."
On Monday, the Prime Minister also urged the US to reconsider its decision to give immunity to the diplomat's wife, saying: "I do not think it can be right to use the process of diplomatic immunity for this type of purpose."
Northamptonshire Police Chief Constable Nick Adderley said US authorities had been appealed to in "the strongest terms" to apply a waiver and "allow the justice process to take place" after Mrs Sacoolas left the UK despite telling officers she did not plan to do so.
Asked about his son by reporters, Mr Dunn became emotional and was supported by his family as he called Harry a "special boy".
He said: "He was a beautiful boy, a beautiful lad, he had so many friends, he loved life, he loved his motorbikes, loved his football.
"He didn't have a bad bone in his body, he just loved his family, he just loved everything.
"He was a special boy and I miss him like mad."
Ms Charles urged Ms Sacoolas to talk to the family face to face.
"Just do the humane thing to do and get on a plane and come back," she said.
"It's so simple, from one parent to another, I still don't see how it can be humanely correct, how could a human just make a decision to get on a plane, literally abscond, run away from what she's done and try to continue her life?" she said.
"What sort of example is she setting to her children?
"She's being dishonest by running away from us."