In 27 cases, about 31 percent of the 2013 exonerations, no crime actually occurred, such as crimes proven later to be accidents or lies by the alleged victim, the report by the National Registry of Exonerations found.
The report said 33 exonerations in the last year were obtained through the initiatives or cooperation of law enforcement. That was the second-highest number of such exonerations, causing the study authors to note "police and prosecutors appear to be taking increasingly active roles in reinvestigating possible false convictions, and to be more responsive to claims of innocence from convicted defendants."
The greatest numbers of exonerations during 2013 were for homicides, 47 percent, and sexual assault, 31 percent. The number of cases not involving those major crimes rose from 4 percent in 2005 to 22 percent in 2013.
The exonerations occurred in several ways, including pardons, dismissal of charges, acquittals on the basis of new evidence and being cleared of charges after their deaths. Texas had the most exonerations, 13.
The registry, a collaboration between the University of Michigan Law School and the Center on Wrongful Convictions at Northwestern University School of Law, lists 1,304 exonerations since 1989.
Since the center began collecting records in 2012, it said it had found a "uniquely high rate" of exonerations for those awaiting the death penalty.
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