WASHINGTON: More than 2,000 people convicted of crimes in the United States over the past 23 years were later found to be innocent, according to a study raising new questions about flaws in the US justice system.

The study, by researchers at the University of Michigan Law School and the Center on Wrongful Convictions at Northwestern University School of Law, compiled information on hundreds of exonerations in the United States, which by Monday had reached 891.

The findings have been compiled at the site exonerationregistry.org.

“The most important thing we know about false convictions is that they happen and on a regular basis,” according to the report by Samuel Gross and Michael Shaffer, both law professors at the University of Michigan.

The findings revealed in the newly-founded National Registry of Exonerations concluded that the huge number of documented false convictions likely was only the tip of the iceberg.

“Most false convictions never see the light of the day,” the researchers wrote.

The researchers found that a quarter of those exonerated of murder convictions had received the death penalty and presumably would have been falsely executed had their convictions not been overturned.

And nearly half of all exonerees convicted of homicide or sexual assault faced either the death penalty or life in prison.

Meanwhile, the study found that those falsely convicted of less severe crimes faced the least chance of ever having their names cleared.

“Most people who are falsely convicted are not exonerated,” Gross said in a statement.

“They serve their time or die in prison. And when they are exonerated, a lot of times it happens quietly, out of public view.”Gross told AFP that capital cases, which are given greater scrutiny than other kinds of convictions, are more likely to result in convictions being overturned.

“Death sentences are about nine times more likely to result in an exoneration than for all homicide convictions,” he said.

“What that must mean is either death prisoners are nine times more likely to be innocent than everybody convicted for homicide -- which would be very shocking -- or it means that 90 percent of innocent people in murder cases are never exonerated,”     he said.

“What it tells you is that in death sentences you spend much more time reviewing the case after the conviction until the execution. Their cases are almost always appealed two or three times,” said Gross.

“More often people are sentenced to life in prison and then they're forgotten,” he said.

The database found that nine of ten of the falsely convicted were men, and about half were African Americans, nearly four times their representation in the overall population, at about 13 percent.

“It's no surprise that black defendants are heavily overrepresented among exonerees,” said the report.

“For sexual assault, the difference is huge, 25% of prisoners but 63% of exonerees were black,” the study said.