According to a new study out Tuesday, in around half their cases, police and prosecutors withheld evidence that would have exculpated them.
Wrongful convictions: US police withhold evidence in many cases
Around 2,500 people have been exonerated of serious crimes after being falsely convicted over the past 30 years in the United States.
The study by the National Registry of Exonerations found that evidence that would have cleared the defendant was withheld in 61 percent of erroneous murder convictions.
And more broadly, 72 percent of exoneration cases in which the defendant was sentenced to death involved misconduct by police and prosecutors.
"Misconduct is generally more common the more extreme the violence," the study says.
The report comes from a joint project of University of California-Irvine, the University of Michigan law school, and Michigan State University law school.
It cites a broad range of police and prosecutor misconduct that contributes to unjust convictions: using questionable techniques to force false confessions, encouraging or coercing witnesses to provide evidence against a defendant; fabricating evidence; and prosecutors skirting the law.
African Americans were slightly more likely than whites to be victims of misconduct leading to false convictions.
But in some types of crime, blacks were far more often falsely convicted. In drug cases, blacks were 12 times more likely than whites to be falsely convicted.
Yet whites were also frequent victims, especially in so-called white-collar crimes involving corruption and fraud.
In such cases, the police were not the problem, but instead zealous federal prosecutors likely seeking to prove themselves with a conviction.
The consequences of injustice can be heavy: the average time spent in jail by a person convicted for murder and later cleared of the crime was 13.9 years, according to the study.
An example is the case of Michael Morton, convicted of murder in Texas in 1987.
The report said county prosecutor Ken Anderson "concealed potent exculpatory evidence that could have cleared Morton and led to the real killer -- who killed another woman in 1988."
Sentence to life in prison, Morton was exonerated by DNA in 2012.
Addressing the problem is difficult, the study admits.
There is a deep, hard-to-change culture in police departments that is focused on getting convictions and resisting criticism, and includes pervasive practices that permit or reward bad behavior, it said.
And prosecutors have unreviewable power on who to charge and for what, and can push the sentences they want.
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