Police probing the death of a 21-year-old activist say the Conservatives have not shared an internal report into bullying despite repeated requests.
Elliott Johnson died in September 2015 on a railway track in Bedfordshire.
An inquest into his death heard he had made bullying claims against a fellow Conservative activist.
The Conservative Party, which investigated those and other claims, maintains it has shared as much information as possible.
The party commissioned law firm Clifford Chance to carry out an investigation into allegations of bullying and the handling of several complaints about a key activist in its youth campaign.
The firm returned its report in August.
British Transport Police is continuing to investigate the death of Mr Johnson, of Wisbech, Cambridgeshire.
A British Transport Police spokeswoman confirmed to the Press Association that it has "made requests for the (Clifford Chance) report but have not yet received it".
Mr Johnson's father Ray said: "They (the Conservatives) feel they are the guardians of the truth in this matter and they are not prepared to divulge what really happened to my son, to us, or even to the police.
"They are hiding behind the very scant guarantee of anonymity to the witnesses.
"The police want to continue their investigation into my son's death and they are being hampered by the Conservative party and the question is why?"
The Conservative Party said it had released "as much of the report as was possible", without publishing information that would lead to the identification of witnesses who gave evidence on condition of anonymity.