The legitimacy of confessions is at the heart of a case of an Arizona woman convicted 23 years ago in the murder of her young son – and of a judge's decision this week to allow her to go free from prison until she can be retried for the murder, possibly without the confession as evidence.

A federal appeals court last spring overturned the conviction of Debra Milke, on grounds that the sole detective who heard her confession, in a closed interrogation room, had lied under oath in other cases and about other confessions – and that his track record should have been disclosed to her defense team during trial. Her alleged confession was not recorded.

Ms. Milke is not yet exonerated – the state plans to retry her on the same murder charges this fall. But her lawyers have already filed a motion to exclude the purported confession from being admitted as evidence.

The whole practice of obtaining confessions has received some black eyes in recent years, as convictions of various kinds – murder, sexual assault, robberies, and so forth – have been overturned when confessions were later found to be false or coerced. Of all exonerations involving false confessions between 1989 and 2012, the majority came in homicide cases, reports the National Registry of Exonerations, a joint collaboration between the University of Michigan Law School and the Center on Wrongful Convictions at Northwestern University School of Law.

“Confession is the most powerful type of evidence, more powerful than eyewitness testimony, because who but a guilty person would ever confess to a crime? The truth is, many innocent people do confess,” says Peter Neufeld, co-director of the Innocence Project, a nonprofit legal clinic in New York City that represents wrongly convicted people.

Juveniles and the mentally disabled are the most prone to giving false confessions, he says, but “ordinary people” are also susceptible if the police believe they have the right suspect and create a threatening enough environment to get an admission of guilt.

News that Milke would be allowed to leave prison pending retrial coincided with reports that the city of Chicago is paying out another $12 million stemming from the ongoing police torture saga involving Jon Burge, a former Chicago police commander who allegedly led a secret unit that is said to have coerced confessions from almost 200 suspects from 1972 to 1991.

Physical torture is “a major factor in giving a false confession,” says Leonard Cavise, a law professor at DePaul University in Chicago and a member of the state commission against torture that reviews cases related to police misconduct.

“What we saw in Chicago with the Burge case is an egregious example of what goes on around the country,” he says. “Usually you don’t see that concentration of torturers on a city’s police force, but individualized torturers exist in other places.”

The city of Chicago has to date paid $85 million in settlements and legal fees stemming from 17 torture cases involving Burge. On Friday, the city paid $6.15 million each to two men who were released from prison four years ago, after spending 21 years in prison before being exonerated. Cook County has spent about $10.7 million.

Some 493 people convicted of homicide were exonerated between 1989 and 2012 – and "false confessions" were cited as a factor in 22 percent of those exonerations, according to the National Registry of Exonerations. (Perjury and false accusations, followed by official police misconduct, are the factors most often cited in such exonerations.)

No physical torture of Milke is alleged to have occurred when she allegedly confessed to ordering the killing of her 4-year-old son in a scheme for an insurance pay-out. Rather, the credibility of her interrogator has come under scrutiny during the appeal of her capital case.

In ruling Thursday that Milke would be allowed to post $250,000 bond, Judge Rosa Mroz of Maricopa County Superior Court noted four cases involving police detective Armando Saldate Jr. that were thrown out on grounds that he had lied under oath. The federal appeals court that overturned Milke's conviction also mentioned other cases that were tossed out because Mr. Saldate had violated the constitutional rights of the suspects.

“No civilized system of justice should have to depend on such flimsy evidence, quite possibly tainted by dishonesty or overzealousness, to decide whether to take someone's life or liberty," wrote the judge with the Ninth US Circuit Court of Appeals in March.

Milke, imprisoned since 1990, she says she is not guilty.

Electronic recordings of police interrogations are a protection against false or coerced confessions, experts say, but most jurisdictions do not require them. Twelve states – Connecticut, Illinois, Maine, Maryland, Missouri, Montana, Nebraska, New Mexico, North Carolina, Ohio, Oregon, and Wisconsin – and the District of Columbia now do mandate such recordings, according to the Innocence Project.

False confessions are often the result of sloppy police work, say those who work to exonerate the wrongly convicted. Investigators usually withhold critical details of a crime to use as a reliability test with suspects, but they can mistakenly volunteer those details in the interrogation room.

“Now we know [in the exonerated cases] that those nonpublic details didn’t originate with the suspect, but were fed to the suspect by the police. They may have done so intentionally, or maybe it happened naturally because they were convinced they had the right person so were just helping them along,” Neufeld says.

California, Texas, and Illinois have seen the most exonerations between 1989 and 2012, according to National Registry of Exonerations data. The ongoing Burge cases account for much of them in Illinois. Three more cases related to Burge are pending. More than 100 cases have yet to be reviewed, Professor Cavise says.

Burge was never prosecuted during his tenure, but he was charged with perjury in a 2010 civil case involving torture. He is serving a nearly five-year federal prison term.