Hospital bosses say privacy laws are preventing them from helping police solve the murder 41 years ago of a 13-year-old girl in South Yorkshire.
Police believe they are tantalisingly close to catching Anne's killer
Anne Dunwell was found in a manure heap near Rotherham on 6 May 1964. She had been sexually abused and strangled.
New forensic evidence shows that her killer had gonorrhoea and may have been treated at a hospital in Rotherham.
But hospital bosses say NHS guidelines on sexually transmitted diseases prevent them naming possible suspects.
Anne's family said they had been told that the list of suspects had been narrowed down to around a dozen names.
The murdered schoolgirl's sister, Irene Hill, said: "It is very frustrating because the police are so near to catching the killer.
Anne's sister Irene Hill has urged hospital bosses to help police
"We don't seem to have the law on our side, the offender seems to have the law on his side.
"We need this information if we are going to solve this crime and this crime needs solving after 41 years."
A Rotherham NHS Trust spokesman said the hospital wanted justice to be done by seeing the murderer caught but their hands were tied by legislation and regulations.
Now Rother Valley MP Kevin Barron plans to take up the case with the Home Office.
"I am sure this is not what Parliament meant when we passed this legislation, that a criminal investigation like this could be frustrated," he told BBC News.
"It might be the case that the person who committed this awful crime is still alive and, if that is the case, they should be prosecuted.
"The family must be really frustrated that the chance of a breakthrough with the advances in DNA science should be blocked because of this."