This week the United States Supreme Court overturned a criminal conviction against a man because it was determined that the prosecution had failed to disclose exculpatory evidence, that is evidence which would have helped the criminal suspect to convince the jury that he was not guilty of the charges. The court said that there was a reasonable probability that if the jury had been aware of this evidence it would have reached a different decision.
In this case there was no physical evidence connecting the suspect to the crime, and only one witness who said that he had seen the suspect commit the crime. With nothing other than the word of that single witness on which to base a prosecution, the reliability of that witness was understandably crucial. In his testimony to the jury the witness said that he had been face to face with suspect during the crime and that he had no doubt that the suspect was the one who had committed it. What neither the jury, nor the suspect knew during the initial trial was that this testimony directly contradicted earlier statements by the witness in which he said he had not seen the perpetrator's face, and that he would not know him if he saw him.
Those statements, in which the witness said he could not recognize the perpetrator, were taken down in police investigators notes. But the prosecution did not disclose those notes to the suspect or his criminal defense team. While its possible that a jury could have decided that the man had been lying in his statements to investigators but telling the truth in his courtroom testimony, that is a decision for the jury not for prosecutors. Prosecutors are required to disclose exculpatory evidence to the suspect.
Source: ABA Journal, "Supreme Court Tosses Murder Conviction for Brady Violation by New Orleans DA," Debra Cassens Weiss, Jan. 10, 2012