Vital information about one of Britain's most prolific serial killers may have been withheld from his trial to ensure that he was hanged, it was claimed yesterday.

Peter Manuel was the third to last person to be hanged in Scotland when he was executed at Barlinnie Prison in Glasgow in 1958, after being convicted of murdering seven people. But although Manuel's was widely considered to be an open and shut case, a legal expert said yesterday that he believed evidence about his mental state was withheld from the court to ensure that he was put to death.

Calling for official archives on the case, some of which are still sealed, to be made public, Richard Goldberg, of the law school at the University of Aberdeen, said that he believed that Manuel could have escaped the gallows if the court had been told the full extent of his health problems, which included a form of epilepsy many believe can cause criminal behaviour.

Dr Goldberg believes that the possibility of a mental disorder was not adequately explored during the trial. He said yesterday: “I think there was considerable evidence that he was a psychopath, there was debate over whether there should be a reprieve, and in my view insufficient weight was given to that evidence and also to the fact that Manuel suffered from temporal lobe epilepsy.

“It is in the public interest that we have access to this information, that the public should see that justice was done properly.”

Manuel was born in the United States but moved to England with his family at the age of 6. He committed some of the worst acts of violence ever seen in Scotland. He had already served jail terms for rape and sexual assault before moving to Glasgow in 1953, but in an orgy of violence between 1956 and 1958 he killed at least seven times, although it is believed that the true figure was higher. Whilst awaiting execution, he is said to have admitted killing up to 18 people.

Manuel was found guilty in May 1958 of the murder of three female members of the Watt family, including a 16-year-old girl, in a house in Burnside, Glasgow, by shooting them in the head at close range. He was also convicted of murdering a family of three in Uddingston, Lanarkshire, as they slept, and 17-year-old Isabelle Cooke as she walked to a school dance.

When detectives later took Manuel to the field where he buried Isabelle and asked where her body was, he replied: “I'm standing on her now.”

The guns he used to murder his victims are still held in the Strathclyde Police museum.

Dr Goldberg, whose father witnessed a medical examination of Manuel while a consultant at a Glasgow hospital, said that he believed Manuel may not have been executed if the full extent of his mental health problems had been examined.

He said that he had come up against a “brick wall” trying to access some files on the case. “There are still files that are closed and there is still uncertainty about what evidence lies there,” he said. Many of the papers used in Manuel's case were sealed for 75 years in 1958.

Dr Goldberg added: “You see the pressure from the Scottish Home Department. They look at his psychopathic personality and they say, 'We don't think he's a psychopath, but even if he is ... he's a very marginal psychopath', so there is a pressure to get him hanged.

“The problem is that psychopathic personality disorder is still is not a basis for a plea of diminished responsibility, unlike in England, and this remains an anomaly,” he added.