Alberta has given up on its prosecution of a woman with an IQ of 50 tricked by Medicine Hat police into pleading guilty to second-degree murder, in a case that raises questions about police tactics and prosecution oversight when suspects with cognitive challenges are involved.

The province's prosecution service stayed legal proceedings against Wendy Scott on Friday, meaning it is discontinuing its case, though it has a year to revive the charge if it wishes. She had been charged in the May, 2011, killing of Casey Armstrong, a 49-year-old father of two, whose body was found in his trailer. In the evidence accepted in court when she pleaded guilty in November, 2012, there was no accusation she had done anything but stand by, and help clean up afterward. Mr. Armstrong had been stabbed repeatedly in the neck, severing his jugular vein and nearly decapitating him.

Medicine Hat police have acknowledged using more than one subterfuge to persuade Ms. Scott to confess to being present at a killing for which they had no DNA evidence and no motive. The police also used tricks to convince Ms. Scott to point an accusing finger at a second woman as the killer, Connie Oakes.

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During the investigation in late 2011 and early 2012, a Medicine Hat police officer falsely told Ms. Scott that her DNA was found in Mr. Armstrong's trailer, and that Ms. Oakes had named her as the killer – another falsehood, according to an Alberta Court of Appeal ruling last spring throwing out Ms. Oakes's conviction.

Police officers are permitted to mislead suspects in some circumstances, and when use of the tactic in the case emerged from the police themselves in new evidence at the appeal court, the judges did not criticize the officer responsible for it. But two of the three appeal judges said the police trickery helped explain why Ms. Scott's testimony against Ms. Oakes was so unreliable. (Ms. Scott, who told police she witnessed the killing, at various times named two men and a woman as the killer, then switched her story to a fourth person, Ms. Oakes, who had a long criminal record.)

The Medicine Hat Police Service, in an e-mail to The Globe and Mail, said they were not aware that Ms. Scott had cognitive problems. "I can say that Wendy Scott did not present herself during the investigation to the investigators as someone who was developmentally delayed. The MHPS Major Crimes Investigators followed recognized Police best practices when conducting this investigation," Inspector Tim McGough said.

But the prosecutor handling the trial of Ms. Oakes was aware of Ms. Scott's cognitive issue. Her testimony was the only evidence identifying Ms. Oakes as the killer, and in closing submissions to the jury, the prosecution not only highlighted her cognitive level, but explained that it made her trustworthy: "Consider Ms. Scott's demeanour, her presentation, her tone, and even her mannerisms. They are childlike. They are real. They are not scripted or disingenuous. And in that regard, she's believable."

A jury convicted Ms. Oakes of second-degree murder in 2013, which brings an automatic life term. A judge set her parole eligibility at 14 years. But the Alberta Court of Appeal threw out the conviction last spring and Alberta gave up on her prosecution, too. "There was no evidence, really," Darryl Royer, a lawyer representing her at her first trial, told The Globe.

In 2015, after Ms. Scott recanted her confession, Legal Aid Alberta refused to fund her appeal – and persisted in that refusal, even after the Crown agreed she had been wrongly convicted. (Legal Aid Alberta declined to comment when contacted for this story.)

Independent Senator Kim Pate is calling for an inquiry into the cases of Ms. Scott and Ms. Oakes that would involve Alberta and federal justice officials. Ms. Oakes had been a volunteer with the Elizabeth Fry Society in Saskatchewan, doing prison advocacy and visits, when Ms. Pate headed that organization. She questioned the actions of prosecutors in Ms. Scott's and Ms. Oakes's initial convictions. "The prosecutors are supposed to be officers of the court, not individuals who are seeking convictions at all costs," she said in an interview.

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Alain Hepner, a Calgary criminal-defence lawyer not involved in the Scott or Oakes cases, said that suspects with learning disabilities, fetal alcohol syndrome or low IQ are vulnerable in the hands of police. "Most see them as authority figures. They figure if the police tell them something, they're going to believe it."

In November, 2012, facing an automatic life sentence for first-degree murder, for which parole eligibility begins at 25 years, Ms. Scott, now in her early 30s, pleaded guilty to second-degree murder, with parole eligibility in 10 years.

Both Ms. Scott and Ms. Oakes followed a similar path: convicted at their trials, they appealed, and won. Both then faced the prospect of a second trial, and in both cases the provincial prosecution service brought in senior prosecutors from outside of Medicine Hat. In Ms. Scott's case, the second prosecution that was just stayed was handled by Douglas Taylor, a senior Crown from the Calgary Rural and Regional Response Office.

"He was brought in because the Executive Director and Chief Crown of Regional Prosecutions wanted a senior prosecutor to view things anew, essentially reviewing the file from the ground up and building a plan to move forward," Katherine Thompson, a prosecution spokeswoman, said in an email.

A stay "was felt to be in the best interests of justice."

Edmonton lawyer Deborah Hatch, who represented Ms. Scott in the appeal, presented evidence to the appeal court that her verbal comprehension was in the 0.1 percentile rank (the lowest one among 1,000 people) and her functional memory at the 0.3 percentile rank. She asked the Court of Appeal to order the province's Attorney-General to pay for her appeal, which it did.

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Mr. Armstrong's family did not return a phone message left with a family member. Joan Blumer, a Calgary lawyer representing Ms. Scott during the second prosecution, said she did not have instructions from her client to speak to the media.