Prison suicide inquiry could cast doubt on Lin Russell murder verdict

Police informant found hanged in his cell in 2002 had told solicitor he wanted to retract false statements he had made in a series of court cases

Detectives have launched an investigation into the death of a police and prison informant who was found hanged in his jail cell. The probe into Paul Day's suicide, shortly after he told a solicitor he wished to retract false evidence he gave in a series of high-profile cases, including the 1996 murders of Lin and Megan Russell, raises the prospect of manslaughter charges and the possible unravelling of some major criminal cases.

Durham police confirmed they are examining the death of Day, 31, from Southend, Essex, who was nearing the end of an eight-year sentence for attempted robbery when he was found dead in the segregation unit of Frankland Prison, Durham, in October 2002.

A five-week inquest – one of the longest ever into a death in custody – heard that in the weeks before he died, Day had been taking part in a dirty protest because of bullying from staff and prisoners on the unit. The jury concluded that Day killed himself after prison staff did nothing to stop prisoners from abusing him, and because of other systemic failures in his care.

It is believed that Day was being bullied after it became known he had acted as an informant in a number of cases including, he claimed, that of Michael Stone, convicted of the murders of Lin and Megan Russell in Kent in 1996.

An investigation by Durham Police began after a complaint from the dead man's parents, Andrew and Pauline Day, about their son's treatment. "We are looking at offences related to Mr Day's death, and our focus is the events as they unfolded at HMP Frankland. We will conduct a thorough and impartial investigation," said detective inspector Eric Malkin from Durham Police.

The Days demanded an independent inquiry after it emerged that the Prison Service's own investigation was seriously flawed and that the inquest into their son's death did not hear crucial evidence about his last days. The Prison Service's investigation was judged to be so poor the then deputy director of the Prison Service, Peter Atherton, commissioned a second one.

Prior to the inquest, despite repeated requests, the Days' lawyers were told that videotapes showing how their son was treated by staff no longer existed. However, during the hearing – after one prisoner said in evidence that he had been beaten "black and blue" by prison staff on the night Day died – the tapes were found. They showed, among other things, Day being strip-searched by several prison officers in riot gear shortly before he died. Frankland's former governor, Phil Copple, was forced to apologise for not finding the tapes sooner.

Last year, the Days discovered that their son had been visited by a lawyer, Michael Gibson of Newcastle-based David Gray Solicitors, six days before his death. At the meeting, Day told Mr Gibson he wanted to retract false statements he had given as an informant in several cases. In a letter to Day, Mr Gibson wrote: "You say you provided false evidence in court on several occasions... You wish to right the wrong suffered by various people against whom you made false statements."

Mr Gibson advised him he could either hand himself in to police or make a formal complaint to the relevant forces. He also suggested writing to the men's solicitors to expose his role. He warned Day that he could face prosecution for perverting the course of justice.

The coroner described the Prison Service's failure to disclose the visit as a "glaring omission". A Prison Service investigation conceded the reports failed to mention the visit – but could not explain why. They denied it had been deliberately withheld.

A Home Office spokesman said the investigation was a police matter. Pauline and Andrew Day declined to comment.