By Agence France-Presse
The majority of the overturned convictions came about as a result of the efforts of "professional exonerators" including prosecutorial Conviction Integrity Units (CIUs) in local district attorneys' offices and non-profits such as the Innocence Project.
The report was published by the National Registry of Exonerations, a joint project of the University of California Irvine Newkirk Center for Science and Society, the University of Michigan Law School, and the Michigan State University College of Law.
"The main reason we're seeing more exonerations now is that they can seek help from innocence organizations and prosecutors' offices who are committed to fixing wrongful convictions and are increasingly working together," said Michigan State University law professor Barbara O'Brien.
"Fifty or a hundred years ago, an innocent defendant in prison had no one to turn to."
In addition to the 139 exonerations, at least 96 other defendants in Chicago and Baltimore were exonerated in "group exonerations" that occurred after the discovery that police officers were systematically framing people for drug crimes.
Experts believe the exonerations likely represent only a small fraction of the total number of miscarriages of justice in the United States.
Many innocent defendants plead guilty to felonies under plea deals in order to avoid the risks of a harsher penalty if they lose their trial.