The National Registry of Exonerations released a report last week that found more prisoners were exonerated in the U.S. in 2015 than in any previous year. The news is largely thanks to an increased number of Conviction Integrity Units (CIUs) across the country that are double-checking convictions to make sure people weren't wrongfully convicted.Currently, there are 24 CIUs at various district attorney's offices across the country. That's more than four times as many as there were in 2011, NBC News reports . But it's still not enough for all of America — where there are more than 6,800,000 people under correctional control since the end of 2014, according to the Bureau of Justice Statistics Of the 149 U.S. exonerations in 2015, 58 were secured by CIUs. The CIU in Harris County, TX, paved the way for 42 exonerations in 2015, while the Brooklyn, NY Conviction Review Unit (CRU) secured eight exonerations in 2015. One defendant, Luis Vargas (pictured), was imprisoned for 16 years before being exonerated of rape charges in November by a Los Angeles county judge.Refinery29 spoke with Mark Hale, chief of the Brooklyn district attorney's CRU, about his department's work, and why there should be more conviction integrity units across the U.S. Hale has worked for the office of the Brooklyn DA for 32 years, and has been chief of the CRU since 2014. Hale works under Brooklyn district attorney Ken Thompson, whom Hale credits with being committed to creating an effective CRU in his district.Hale explained to Refinery29 that his department investigates all levels of felony crimes, not just homicides. In the nationwide 2015 exoneration report (PDF), 58 of the 149 exonerees were exonerated from homicide charges, including 54 who were exonerated for murder, and four who were exonerated for manslaughter."Over the time that I've been doing this, we have received somewhere in the neighborhood of 500 petitions, or inquiries," Hale told Refinery29. Of those 500, his team has initiated full investigations into more than 100 of the cases — leading to 17 exonerations.Exactly how do conviction integrity units decide which cases to review? Hale told Refinery29 that individuals, whether lawyers, family members of the convicted, or the defendants themselves, contact his department about wrongful convictions. The unit then does an initial investigation to see if the claim is credible — and if it is, the unit will assign one of eight assistant district attorneys to investigate the claim further.CIU investigations involve re-interviewing witnesses, speaking with witnesses who weren't called on during the original trial, and re-examining evidence. "We're pretty rigorous about going to a lot of lengths to find these things, and to find these people," Hale told Refinery29.For Hale's unit, his team sends their investigation results to an independent review panel of three attorneys not involved with the cases, who then recommend to the district attorney whether or not the conviction should be upheld."What we're doing is an exercise of justice, that is, making sure that we have the right person, and that they were convicted in the right fashion," Hale told Refinery29. "If either of those things didn't happen, obviously, we're going to vacate their conviction."And while units like Hale's don't take demographics into account when considering which petitions to look into, the 2015 exonerations report paints a grim picture of the United States' conviction record. Of the 58 homicide exonerees listed by the National Registry of Exonerations in 2015, 50% of them were Black.