The American Bar Association is calling for a nationwide freeze on executions.
The group says serious problems in state death penalty systems compromise fairness and accuracy in capital punishment cases.
Problems cited in an ABA report include spotty collection and preservation of DNA evidence; misidentification by eyewitnesses; false confessions; and persistent racial disparities making death sentences more likely when the victims are white.
The report is a compilation of separate reviews of how the death penalty operates in eight states: Alabama, Arizona, Georgia, Florida, Indiana, Ohio, Pennsylvania and Tennessee.
Prosecutors and death penalty supporters have said the eight state studies were flawed because the ABA teams were made up mainly of death penalty opponents.