The family of teenage motorcyclist Harry Dunn have said they are disgusted and "not sleeping well" after the Crown Prosecution Service "refused" to meet with them.
The CPS were handed the completed file of evidence in the 19-year-old's case on November 1.
The PA news agency understands the family have requested two meetings with Chief Crown Prosecutor Janine Smith - and have been told a meeting will "not be possible".
The chief prosecutor has said she will meet with the family once a chargingdecision has been made.
Harry's stepfather Bruce Charles has said the "delay" in a charging decisionis "eating away inside us".
He told PA that the family wants a "100% cast iron promise" of no externalinterference in the case in a face-to-face meeting with a CPS representative.
The family's spokesman has said they will now seek an "urgent" meeting with the Director of Public Prosecutions.
Harry was killed when his motorbike was involved in a head-on collision with a car outside RAF Croughton in Northamptonshire on August 27.
The suspect in the case, 42-year-old Anne Sacoolas, claimed diplomatic immunity after the crash and was able to return to the US.
Speaking of the family's frustration over the CPS's decision not to meet withthem, Mr Charles told PA: "Our family have enormous respect for the CPS and the valuable work they do on the public's behalf.
"We know prosecutors work incredibly hard each and every day, and we have no wish to undermine the reputation of the institution.
"However, as the whole nation now knows, we have felt totally unsupported by the authorities since Harry died and the delay in charging in this most simple of cases is just eating away inside us."
Mr Charles continued: "None of us are sleeping well and we are convincedsomething is up and the governments are interfering.
"We simply want to meet with the prosecutor to look her in the eye and to geta 100% cast-iron promise that there is no external interference and to understand what on earth is causing the delay.
"We watch the news every day and see that people are charged quickly in other high-profile, far more complex crimes. Why isn't that happening in our simple case?"
He added: "We have a right to know and are being fobbed off yet again. We are disgusted at the prosecutor's latest refusal to meet us and her persistentstatements that she cannot tell us when the charge will be authorised.
"We are sick and tired of her telling us how sorry she is for our loss,offering condolences and understanding our frustration. They are just emptywords."
Commenting on the CPS involvement in the case, the family's spokesman Radd Seiger said: "We know Northamptonshire Police let the family down and, to their credit, have acknowledged that, and wish to work with the family to learn the lessons.
"It may be that the CPS are doing a sterling job, but the important point isthat the family feel that that is not the case for perfectly understandable andvalid reasons as supported by thousands of like-minded messages the family have received.
"Anybody in their shoes in would feel exactly the same way, even theprosecutor and the politicians.
"The CPS appear not to be taking any account of the mental torture the family are going through and their persistent refusal to agree to the early meeting the family are entitled to is unconscionable and incomprehensible.
"The parents have therefore asked me to seek an urgent meeting with theDirector of Public Prosecutions to take the issue up on their behalf and I willbe making that call this morning."
A CPS spokeswoman said: "We appreciate how difficult this time must be forHarry's loved ones and understand that his family are seeking answers.
"However, whilst the case is under active consideration, we are unable to meet with Harry's family to discuss its progress.
"The CPS is carefully considering all the available information, including theimpact on Harry's family, in order to make an independent and objective charging decision.
"Our Chief Crown Prosecutor will meet with Harry's parents as soon as is itpossible to provide them with an update."