The number of U.S. convicted criminals exonerated reached an unprecedented high in 2014. The National Registry of Exonerations revealed on Tuesday that last year saw 125 criminals exonerated. 

Reuters reports this is due in part to prosecutors admitting their offices made mistakes. For example, The Registry is part of a project by the University of Michigan Law School and has tracked exonerations since 1989.

The study reported the most exonerations occurred in Texas, New York and Illinois.

A prisoner's convictions may be overturned in cases that provide DNA clearing a person of a crime, witnesses recanting testimony or committing perjury.

More than half of the wrongful convictions in 2014, at least 67, were obtained with the cooperation of law enforcement. Many of these were achieved through "conviction integrity units" set up by prosecutors who wanted to review questionable cases.  

The number of these integrity units have grown since the first in 2002 to at least 15 last year. The most active were in Houston and Brooklyn, which assisted in 39 combined wrongful convictions. These ranged from murder to drug offenses.

In a case in Ohio, Ricky Jackson spent 39 years in prison for murder. He was the longest held U.S. prisoner who was later exonerated. He was freed last November after a witness admitted he didn't see the crime occur.

In Chicago a judge overturned charges against Alstory Simon after spending 15 years in prison for a double murder. Another man, Anthony Porter, was convicted of the same crime in 1983 and was released after Simon confessed to the murders. After re-investigating the crime, it was revealed Simon's confession was coerced.

In 46 percent, 58, of the 125 exonerations last year, no crime actually occurred.

Murder cases were a substantial portion of wrongful convictions standing at 38 percent, 48, of the convictions.

A large number of exonerations also involved 33 drug cases from the Houston area. Prosecutors found crime lab analysis revealed negative results for illegal drugs after defendants had already taken plea deals.