A new study has found that Harris County leads the country in exonerations, turning loose 48 people in 2016 alone. That's because its crime labs take an added precaution most others don't: testing the materials seized from drug defendants even after they enter guilty pleas.

And when the supposed drugs they possessed were tested, in many cases no illegal drugs were found. According to a study released Tuesday by the National Registry of Exonerations, Harris County had all but 10 of the state's 58 exonerations last year. The state with the second highest number, Illinois, had 16 exonerations in 2016.

Harris County District Attorney Kim Ogg said her department isn't ashamed of the high number. Ogg said her department takes extra precautions "to ensure the integrity of our convictions,” a move she hopes more crime labs across the nation will follow.

“We take the Supreme Court mandate to disclose favorable evidence, even post-conviction, to heart,” Ogg told the Texas Tribune. “We’ve assigned resources to making sure that even in cases where someone has pleaded guilty, but did so without actually being guilty, that we take every measure to make that right.”

Most of those convicted and later exonerated for drug-related offenses in Harris County were African-American. The researchers found that African-Americans are roughly five times as likely to go to prison for drug possession as whites. And judging from exonerations, innocent black people are about 12 times more likely to be convicted of drug crimes than innocent white people, the study found.

“Harris County is extremely valuable for our research because it’s an unusual example of something you wouldn’t otherwise see,” said Samuel Gross, a University of Michigan law professor and senior editor of the study. "One possibility is that they're more conscientious. What's striking is that they do this."

Gross said the rate of illegal drug use is roughly the same for whites and blacks, but the number of arrests and convictions is much higher for African-Americans than for whites.

“Why are African-Americans so overrepresented when it comes to people who are falsely arrested for drug possession?" Gross said. "The answer is that drug law enforcement bears more heavily on the black community than the white community.”

The study also found:

African-American prisoners who were convicted of murder are about 50 percent more likely to be innocent than other convicted murderers.

Innocent black people are about 12 times more likely to be convicted of drug crimes than innocent white people.

62 percent of those exonerated for drug crimes in Harris County in 2016 were African-American — in a county where they make up 20 percent of the population.

“Historically, our drug laws have been enforced unequally, and I want to stop that,” Ogg said. “Obviously that’s going to require law enforcement leadership, training and policies that effect the way our laws are enforced, but as DA I can level the playing field by offering an equal opportunity to all offenders.”

The number of African-Americans arrested for drug-related offenses came as no surprise to Jay Jenkins, a project attorney for the Texas Criminal Justice Coalition who has worked in Harris County for nearly three years. Jenkins said that oftentimes defendants will plead guilty to a drug charge to try to get out of jail.

“A lot of those folks plead guilty before the lab has even gotten the test back to the court,” Jenkins said. “Low-income defendants have very little options if they can’t make bail.”

While Jenkins commended the Harris County officials for testing suspected drugs even after a defendant has pleaded guilty, he said there’s work to be done when it comes to how law enforcement treats the African-American community.

“The way that these communities are policed and the way that these individuals are prosecuted — it really is different than how they’d police a white person from a rich neighborhood,” Jenkins said. “They almost get an entirely separate criminal justice system from what is used to police and prosecute low-income communities here in Houston.”

Ogg said she'll continue to work to level the playing field on drug-related cases.

“I think the big takeaway for us is that every case of wrongful conviction has to be seriously considered because it shapes the public’s trust in our system,” Ogg said. “I promise to help try and build a trustworthy system ... These exonerations on these drug cases are part of that.”