WASHINGTON (AP) — Science and law have led to the exoneration of hundreds of criminal defendants in recent decades, but big questions remain: How many other innocent defendants are locked up? How many are wrongly executed?

About one in 25 people imprisoned under a death sentence is likely innocent, according to a new statistical study appearing in the Proceedings of the National Academy of Sciences. And that means it is all but certain that at least several of the 1,320 defendants executed since 1977 were innocent, the study says.