Shocking: How Faulty Science Lands Innocent People Behind Bars as Accused Child Murderers

This story was produced in a collaboration between PBS Frontline, NPR and ProPublica. 

Her name was Isis Charm Vas and at 6 months old she was a slight child -- fifth percentile in height and weight.

When the ambulance sped her to Northwest Texas Hospital on a Saturday morning in October 2000, doctors and nurses feared that someone had done something awful to her delicate little body.

A constellation of bruises stretched across her pale skin. CT scans showed blood pooling on her brain and swelling. Her vagina was bleeding, as well. The damage was so severe that her body's vital organs were shutting down.

An autopsy bolstered the initial suspicions that she'd been abused. Dr. Joni McClain, a forensic pathologist, ruled Isis' death a homicide and said the baby had been sexually violated. McClain would later describe it as a "classic" case of blunt force trauma, the type of damage often done by a beating. The police investigation that followed was constructed almost entirely from medical evidence. In the end, prosecutors indicted one of the child's babysitters: Ernie Lopez.

Today, Lopez is serving a 60-year prison term for sexual assault and is still facing capital murder charges.

But in the years since Lopez was sent to the penitentiary, a growing body of evidence has emerged suggesting that McClain and the hospital staffers were wrong about what happened to Isis -- and that her death was not the result of a criminal attack.

If Lopez is ultimately exonerated, his case will not be unique. An investigation by ProPublica, PBS "Frontline" and NPR has found that medical examiners and coroners have repeatedly mishandled cases of infant and child deaths, helping to put innocent people behind bars.

We analyzed nearly two dozen cases in the United States and Canada in which people have been accused of killing children based on flawed or biased work by forensic pathologists, and then later cleared.

Some spent years in prison before courts overturned their convictions. In 2004, San Diego prosecutors moved to dismiss charges against a man who'd been imprisoned for two decades for murdering his girlfriend's son.

Others were freed more swiftly but endured hardships nonetheless. An El Paso, Texas, jury acquitted a woman of killing her child in 2010, but after spending 22 months in the county jail, she still had to wage a legal battle to regain custody of her other children.

The questionable prosecutions identified in our investigation had common elements:

Often, authorities had little to go on other than autopsy findings. Many of the doctors who conducted post-mortem examinations failed to consult specialists in childhood injuries or ailments, or to thoroughly review medical records that could have affected their conclusions. In several cases, forensic pathologists worked so closely with authorities, they effectively became agents of law enforcement, rather than objective arbiters of scientific evidence.

Some experts in the field say worries about mistakes in child death cases are overstated. "The vast majority of forensic pathologists recognize a child abuse case when they see it, and it's not because they want to persecute people," said Dr. Mary Case, chief medical examiner for four Missouri counties including St. Louis County.

But others say the criminal justice system has yet to confront the full scope of the problem, and that, as a result, more innocent people may be serving time for crimes they didn't commit. "I think it's time to look at these cases again," said Dr. Michael Laposata, chief pathologist at Vanderbilt University Medical Center, adding that this could "result in the liberation of a number of falsely accused people."

Lopez, 40, a soft-spoken man with a slight twang, still can't quite believe he may spend the rest of his life locked up for something he says he didn't do: harming the infant he nicknamed "Little Bird."