WASHINGTON — More than 2,000 people have been freed from prison since 1989 after they were found to have been wrongly convicted of serious crimes, according to a new National Registry of Exonerations compiled by University of Michigan Law School and Northwestern University.

Its sponsors say it is by far the largest database of such cases, and they hope it will help reveal why the criminal justice system sometimes misfires, prosecuting and convicting the innocent.

“The more we learn about false convictions, the better we’ll be at preventing them,” said Samuel Gross, a University of Michigan law professor.

The registry covers the period since DNA came into common use and revealed, to the surprise of many prosecutors and judges, that a significant number of convicted rapists and murderers were innocent. The Innocence Project in New York says DNA alone has freed 289 prisoners since 1989.

Criminal law experts have been studying the growing number of exonerations. Some cases have involved police corruption or witnesses who recanted. Experts have also pointed to faulty eyewitness testimony and lying witnesses as common problems.

Beyond that, a surprising number of cases involved suspects who confessed to crimes they didn’t commit.

“Nobody had an inkling of the serious problem of false confessions until we had this data,” said Rob Warden, executive director of the Center on Wrongful Convictions at Northwestern University. Under persistent and prolonged questioning by investigators, some suspects confessed to crimes such as rape, even though DNA later revealed they were not the perpetrators.

Among the states, Illinois has the most exonerations listed in the new registry, and among counties, Cook County and Chicago led the way, followed by Dallas and Los Angeles. However, the sponsors of the new registry do not contend that their data permits strong comparisons across counties or states because only about 900 of the cases were examined in detail by jurisdiction.

“It’s clear that the exonerations we found are the tip of the iceberg,” Gross said.

For example, several counties in California with more than 1 million residents, including San Bernardino and Alameda, listed no exonerations. By contrast, Cook County had 78 and Dallas County 36.

“Obviously there are false convictions in those [other] counties. We just don’t know about them,” he said.

The figures are also constantly changing. Last week, shortly after a report on the registry was completed, prosecutors in Lake County, Ill., dropped sexual assault charges against Bennie Starks. He had been convicted of the 1986 rape of an elderly woman and had served 20 years in prison. DNA evidence taken from the victim pointed to a different man.

Updating the registry, Warden said Illinois now had 103 exonerations.

david.savage@latimes.com