It is almost seven years since Susan May walked out of Askham Grange prison in North Yorkshire after serving 12 years for a murder she insists she did not commit. Despite all those years of incarceration, there was a part of her that wasn't ready to walk free. "I came out with a heavy heart because I always thought I would only get out of prison with my conviction overturned," she said.

Ever since her release, May has devoted herself to proving her innocence. Now the Guardian has discovered evidence suggesting police ignored vital clues supporting May's case that she did not suffocate her 89-year-old aunt, Hilda Marchbank, in 1992, in Royton, Oldham, Greater Manchester.

The Guardian has discovered that Greater Manchester police failed to follow up evidence pointing to a man whom police said was a "good suspect", whose name was given to detectives in an anonymous phone call shortly after the murder. Michael Rawlinson, a heroin addict, had convictions for burgling the homes of elderly people. In 2001, six years after Marchbank's death, he was killed in a drug-related dispute.

The Guardian has also uncovered claims that police tried to persuade a witness to lie, apparently to eliminate other leads from the inquiry and concentrate on building a case against May. The witness, George Cragg, has been tracked down by the Guardian 20 years after the killing.

Campaigners for May say police seem to have instinctively believed she committed the crime ? despite a lack of motive and a dearth of evidence to prove she beat her aunt around the face and suffocated her with her own pillow.

May was convicted on the flimsiest of evidence, the campaigners say, comprising mainly three marks, said to be her fingerprints, that allegedly contained her aunt's blood. Fresh doubts have emerged in recent years about the testing method, about whether the marks are May's fingerprints ? and even whether they contained human blood.

'Good suspect'

For the detectives investigating the murder of Hilda Marchbank, it ought to have been a significant breakthrough: a near neighbour of the victim had seen a red Ford Fiesta outside the victim's house at midnight on the night she was killed in March 1992. Although the car was unoccupied, its engine was running for 15 minutes.

Hilda Marchbank was found murdered at her home in Royton, near Oldham, Greater Manchester. Photograph: Manchester Evening News Syndication

Another neighbour had seen the car drive away soon after and gave a good description of three men in the vehicle. Marchbank was found next morning by May, her niece and carer, who was then 48.

The Guardian has learned of a further link to the red car: two days after the murder, an anonymous caller gave police the names of two men, alleging they were the killers. Another anonymous caller, who rang six days before the murder, reported both men robbing an elderly woman in the area two weeks earlier.

It transpired such a robbery had been committed and the details matched those given in the call. Rawlinson, one of the men named, had access to a red Fiesta, owned by his sister. The car was sold three days after the murder.

Detectives knew Rawlinson and the car were suspicious: in police files, he was described as a "good suspect". But Rawlinson was never treated as a serious suspect, according to May, who found herself accused of murdering the aunt she says she have loved and looked after. She claims police quickly made up their minds that she was responsible for the murder.

Their reasoning? May had been in a clandestine affair with a local man but, when questioned by police, denied the relationship. "It was none of their business and nothing to do with Auntie's death," is how she explains the lie that she believes led to her serving 12 years of a life sentence for murder.

The prosecution had tried bring up the "lavishing of presents" on the man she was having an affair with, using Marchbank's money. The judge told the jury to forget that claim, saying there was no evidence. May had power of attorney over her aunt's finances.

The police, far from following up the lead on Rawlinson and his possible accomplices, seem to have concentrated on eliminating him from their inquiries.

They traced the owners of red Ford Fiestas in the Oldham and Rochdale areas. Their investigation led them to an Oldham man, Craig Turner. Coincidentally, Turner and two friends, George Cragg and Robin Walker, had been in the vicinity of the murder scene on the evening of the killing, 11 March.

Two weeks after the murder, the three men were interviewed at Oldham police station. When tracked down by the Guardian recently, Cragg ? now a Blackpool hotelier ? said he and his friends were told from the start they were not suspects. Instead, they were asked about their movements on the night of 11 March.

They said they had gone in Cragg's white Ford Granada to a house near Marchbank's home to see a plumber who owed Cragg money. It wasn't the answer the police wanted, it seems. Cragg says police told him they needed to "eliminate the red Fiesta, we know who's done it, she's done it". He now believes police were referring to May.

Cragg says two weeks later, two detectives came to his house and asked him again what car he had been driving when he visited the plumber. He repeated that he had been driving his white Granada. One officer, he says, then said: "Can you not say you were driving a red Fiesta?" Asked why they would want him to say this, one officer is alleged to have replied: "We have her bang to rights and we want to eliminate the red Fiesta."

Cragg's wife, Julie, was present throughout this visit and says her husband's account is accurate. She says police "propositioned" her husband to lie, though he refused.

The couple's statements have been passed to the Criminal Cases Review Commission (CCRC), which has invited May to resubmit her application for the case to be referred back to the appeal court.

Six weeks after the murder and a fortnight after May had been charged, police traced the red Fiesta belonging to Rawlinson's sister. Forensic samples were taken from the vehicle but never sent for laboratory testing. Years later, when the CCRC asked why May's lawyers were not told this car had been found, they were told the file had been placed in the wrong folder.

Rawlinson was questioned about his movements on the date of the murder. He said he and his girlfriend had been drinking at the house of a male friend from noon on 11 March until noon the following day. He said they had not left the house, watched TV or listened to the radio. His girlfriend and the man backed his alibi and the matter was left there.

Six years later, Rawlinson was murdered. In 2001, Campbell Malone, then May's solicitor, paid a prison visit to Gary Brierley, the man convicted of killing Rawlinson. According to Malone, Brierley said when he was arrested, police in Rochdale told him he "deserved a medal" because Rawlinson had "killed an old lady".

May's trial lawyers knew nothing of the sighting of the red car and its link to Rawlinson, nor of the anonymous phone call naming him as the killer. The trial jury, at Manchester crown court in 1993, was left equally in the dark. It found her guilty.

In all her 12 years inside, May never wavered in her protestations of innocence. She refused to comply with offending behaviour programmes and other measures that lifers must normally follow in order to apply for parole.

But in 2005, May was released, a week before finishing the minimum tariff of 12 years ordered by the judge, becoming the first prisoner to gain release "on time" while still denyi
ng the offence.

The former deputy head of Hampshire CID, Des Thomas, has written three critical reviews between 2009 and last year of the police investigation into the death of Marchbank. He said: "The evidence discovered by the Guardian may point to a pattern of behaviour which raises a reasonable suspicion that the investigation was prejudicial, if not overtly corrupt.

"My own review, based on disclosed documents, revealed a number of apparent anomalies in the case. Combined with evidence located by the Guardian, these anomalies may point to an investigation, the principal purpose of which was to prove, by the selective use and non-disclosure of evidence, the hypothesis that Susan May was guilty of murdering her aunt. The original investigation report records [that] the red car and its occupants were never traced. This appears to be in direct contradiction to the evidence recovered by the Guardian."

In his 2009 report Thomas said a "disinterested observer may conclude that some evidence had been manipulated to construct a case against Susan May". He added that a "number of police witnesses may have adjusted their evidence to fit a desired rather than valid outcome."

A spokesman for Greater Manchester police said the force would not be putting any of the officers who investigated the murder of Marchbank forward to comment. He said the case was tried at crown court and had been the subject of judicial appeals, which upheld the conviction. "The criminal justice system has been successively satisfied of her ? May's ? guilt and there is nothing further to say on the matter," he added.

Flawed evidence

May will also be asking the CCRC to consider new forensic evidence relating to marks on the wall of her aunt's home. Arie Zeelenberg, the former head of the fingerprint service at the Dutch national police agency, believes forensic evidence that helped convict May was flawed and is conducting a review on her behalf.

In 2007, Zeelenberg advised the inquiry into the Scottish Criminal Records Office's fingerprint bureau, following the case of former policewoman Shirley McKie, who, in 2006, was awarded £750,000 from the Scottish Executive after being wrongly accused of leaving her fingerprint at a murder scene and lying about it. Professor Allan Jamieson, director of the Forensic Institute, has also lent his name to the challenge to the evidence that helped convict May.

Now 67, May survives on a small state pension and is in remission from breast cancer she feels was exacerbated by her incarceration. When she was released, she was unable to get work ? "my conviction came up on every Criminal Records Bureau check" ? and instead devoted her life to clearing her name.

She is extremely thankful to the Craggs for speaking out. She said: "Despite 20 years passing since Auntie was murdered, I still firmly believe there are individuals who know something which could help clear my name and bring long awaited justice for my auntie. I may be free from prison, but remain locked up in this wrongful conviction and will never stop fighting to clear my name."

May's case has twice been rejected by the court of appeal, most recently in 2001. Last year, the CCRC refused to refer her for a third time. Now, with the evidence supplied by the Guardian about Cragg's claims about the meeting and other new material, the commissioners have invited her to resubmit her claim.

May's mother ? Hilda Marchbank's sister ? died about a year after the conviction. With other relatives, she supported her daughter wholeheartedly.

Michael Mansfield QC, who represented May during her second appeal in 2001, has written to the CCRC underscoring his "firm belief" in her innocence and asking the commission not to close its door on her.

"I loved my auntie," May told the Guardian. "The worst thing has been the idea people believed I could have hurt her."

Additional research by Mischa Wilmers