The mother of Harry Dunn has accused the British foreign secretary of misleading her over a potential investigation into the motorist suspected of involvement in the collision that killed her son.

Dunn, 19, died in August when he collided with a car allegedly being driven by Anne Sacoolas, the wife of a US diplomat, who then left the UK under diplomatic immunity.

His mother, Charlotte Charles, said she was angry about the mixed messages she felt she had received from the authorities, including from the foreign secretary, Dominic Raab.

“A month ago today [Raab] said in parliament there were no obstacles to justice, but yesterday he stated he was still working to clear obstacles,” she told the BBC’s Victoria Derbyshire show on Thursday.

“I’m just really angry that we were led to believe there was nothing stopping the [Crown Prosecution Service] looking at Harry’s case and making that decision, and yesterday morning Dominic Raab just blew that open again for us.”

She said it should be an “extremely straightforward” case since the incident was captured on CCTV .

On Wednesday, Raab said the Foreign Office was “doing everything it can to clear the path for justice for Harry’s family” and allow the police and the Crown Prosecution Service to “do their job”.

However, on 21 October he told the House of Commons: “At every stage in this process we have sought to clear away any obstacles to justice being done … I am currently aware of no barriers to justice in this case.”

Dunn’s parents have said they intend to take legal action against the Foreign Office, but the government department has said it would “seek costs” for any judicial review brought. Charles said this was “disgusting”.

Radd Seiger, the family’s spokesperson, told the Guardian that the family was being treated “like dirt” and was “deeply suspicious” of the government.

“Either Raab on 21 October was lying to the House of Commons when he said there was no obstacle to justice or he was lying yesterday,” he said. “The family is deeply suspicious and concerned that any delay in charging the suspect is as a result of interference from either the FCO or the Trump administration.”

He added: “This family of British citizens have been treated like they’re the dirt on the Foreign Office’s shoes.”

A Foreign Office spokesman said: “We have deep sympathy for Harry’s family. We have done and will continue to do everything we properly can to ensure that justice is done. As the foreign secretary set out in parliament, the individual involved had diplomatic immunity whilst in the country under the Vienna convention on diplomatic relations.”

The spokesman said it was “usual government practice to seek costs in legal challenges of this kind”.

It is understood that the next steps in the case are a matter for the CPS, which conducts criminal prosecutions, after Northamptonshire police officers interviewed Sacoolas in the US and submitted a file.

A CPS spokesperson said: “We fully understand how difficult this must be for Harry Dunn’s family, and the CPS is doing everything possible to reach a charging decision as soon as we can. Unfortunately, we are unable to tell the family how long that will take. Each case is different and the CPS has a duty to carefully consider all the available information in order to make an independent and objective decision.”