At least 139 convicted defendants in the United States were exonerated last year, and most owe it to the work of lawyers in prosecutors’ offices and private organizations dedicated to finding wrongful convictions.

Such “professional exonerators” were responsible for more than half of the exonerations in 2017 and have been a driving force in overturning wrongful convictions in recent years, according to a new report from the National Registry of Exonerations, which tracks such cases.

“It makes you really wonder what would the feelings on exoneration be, and how many would we see, if there were more of these organizations,” said Barbara O’Brien, a law professor at Michigan State University and the editor of the registry.

Since 1989, when DNA was first used in an exoneration, at least 2,100 people have been cleared of their convictions, according to the registry, underscoring the fact that the system sometimes gets things wrong.