There are many reasons why wrongful conviction is a persistent problem in the United States, including some we have discussed recently (false confessions, for instance). And while wrongful convictions are often the result of shoddy work by police and prosecutors, the mistakes are usually honest ones.
Unfortunately, however, wrongful convictions can also be the result of prosecutorial misconduct. This is when prosecutors knowingly do something illegal, or neglect to do something they are required to do, in pursuit of a conviction. One example of misconduct is withholding evidence that could demonstrate the defendant's innocence (also known as exculpatory evidence).
Such acts are often referred to as "Brady violations" because of the 1963 U.S. Supreme Court ruling in Brady v. Maryland. For purposes of this post, the details of that case are less important than the precedent set by the ruling. In short, the Supreme Court's ruling means that if prosecutors are in possession of or are aware of exculpatory evidence, they have a legal duty to disclose that evidence to the defense.
Subsequent court decisions after the Brady ruling essentially extended the obligation to police officers and investigators. Law enforcement officers have a duty to turn over potentially exculpatory evidence to prosecutors, who then decide if it needs to be turned over to the defense.
Another case in 1972 known as Giglio v. United States further expanded Brady obligations. Because of this ruling, prosecutors are required to turn over evidence that could discredit a witness. This includes police officers who testify.
For instance, the defense has a right to know if the testifying witness was offered a reduced sentence or some other benefit in exchange for their testimony. Additionally, if a testifying police officer has previously been disciplined for falsifying reports or engaging in other dishonest behavior, this must be shared with the defense.
The idea that prosecutors would withhold exculpatory evidence in order to knowingly convict an innocent person is unconscionable, but it happens. Experienced criminal defense attorneys work hard to not only defend accused clients but also to overturn wrongful convictions related to Brady violations.
Source: Public Agency Training Council, "Brady v. Maryland: Do you Understand your Obligations?" Steve Rothlein