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A record-breaking number of people were exonerated in 2015 — freed after serving time in American prisons for crimes they did not commit.

In all, 149 people spent an average of 15 years in prison before being cleared last year, according to a new report (.pdf) out Wednesday from the National Registry of Exonerations, a project at the University of Michigan Law School.

The convictions ranged from lower level offenses, such as 47 drug crimes, to major felonies, including 54 murder convictions that were overturned. Five of the convicts were awaiting execution, and were saved last year when courts ruled they didn't belong in the prison in the first place.

Luis Vargas, who was in prison for 16 years, reacts in court as he is exonerated Monday, Nov. 23, 2015. Francine Orr / Los Angeles Times via AP

Of the people wrongly convicted for homicides, the report notes, "more than two-thirds were minorities, including half who were African American."

Twenty-seven of the innocent convicts falsely confessed to their crimes, a group comprised mainly of children or the mentally handicapped, according to the report.

Falsely Accused

"I feel that if I die this afternoon, I’ll be able to go to my tomb and rest in peace because my name has finally been cleared," said William Vasquez, who was cleared in December of an arson that killed a mother and her five children in 1981.

Vasquez already served 31 years in prison, where he went blind for untreated glaucoma, and was released in 2012. He even had an alibi witness at trial, who testified he was with her when the fire was set, but the building owner, Hannah Quick, accused him of the arson. She recanted shortly before her death in 2014.

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An attorney who worked on the case, Adele Bernhard, credited the Brooklyn District Attorney's office for reviewing the conviction.

"We know very little about the sorts of mistakes we make ... how frequently they happen."

“I was impressed with the sheer dedication and resources that they brought to bear,” she said.

Brooklyn D.A. Ken Thompson has prioritized a Conviction Integrity Unit in his office to proactively address false convictions, which, Thompson says, undermines "the public’s confidence and trust in our criminal justice system."

Prosecutors Playing Defense

The notion that prosecutors would spend time double-checking past convictions is quite new. Traditionally, most prosecutors view their role as convicting at trial and defending victories in virtually all appeals — not reassessing whether evidence undermines a conviction their office once sought.

The first Conviction Integrity Unit was created by a California district attorney in 2002, and 24 D.A.'s now have such units, a fourfold increase since 2011.

In places where the units have been created, the numbers show that when the government joins the cause of looking for wrongful convictions, they are not hard to find.

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For example, about 28 percent of all exonerations last year came from a single office, Harris County, Texas, because it has rigorously reviewed past convictions. Prosecutors found that their previous, aggressive tactics — threatening defendants with long prison terms to secure guilty pleas — led some defendants to plead guilty to drug crimes that never even occurred.

Harris County's review opens "a window into the world of plea bargaining in misdemeanors," the report states, noting the local prosecutor now forbids aggressive plea bargaining tactics unless, lab tests confirming a drug crime happened in the first place.

University of Michigan law professor Samuel Gross, who runs the school's exoneration registry, says this year's record breaking numbers shows some "slow, steady" shift towards taking exoneration seriously in parts of the country.

“We know very little about the sorts of mistakes we make," he adds, or "how frequently they happen."

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The mounting interest in the exoneration process among some D.A.s comes amidst a renewed cultural and political assessment of America's criminal courts.

Leaders in both parties now criticize a justice system they say is overly punitive, while shows like "Making a Murderer," the Netflix documentary tracking a man exonerated for one crime only to be convicted of another, have sparked a passionate debate about certain prosecutorial tactics and the true meaning of reasonable doubt.

The 30-page University of Michigan report — mostly a dry analysis of conviction policy — notes at one point that "Making a Murderer" both "reflects and contributes" to a growing awareness that wrongful convictions are a real problem.

Unlike media or movie treatments of the issue, however, prosecutors and judges don't have the luxury of fixating on one particularly dramatic case. In a system where only 6 percent of convictions actually have evidence tested at trial — the rest are plea bargained – the question is how to double-check cases that may not have ever been fact-checked in the first place.