Prosecutors are the most powerful players in the American criminal justice system. Their decisions — like whom to charge with a crime, and what sentence to seek — have profound consequences.

So why is it so hard to keep them from breaking the law or violating the Constitution?

The short answer is that they are almost never held accountable for misconduct, even when it results in wrongful convictions. It is time for a new approach to ending this behavior: federal oversight of prosecutors’ offices that repeatedly ignore defendants’ legal and constitutional rights. There is a successful model for this in the Justice Department’s monitoring of police departments with histories of misconduct.

Among the most serious prosecutorial violations is the withholding of evidence that could help a defendant prove his or her innocence or get a reduced sentence — a practice so widespread that one federal judge called it an “epidemic.” Under the 1963 landmark Supreme Court case Brady v. Maryland, prosecutors are required to turn over any exculpatory evidence to a defendant that could materially affect a verdict or sentence. Yet in many district attorneys’ offices, the Brady rule is considered nothing more than a suggestion, with prosecutors routinely holding back such evidence to win their cases.

Nowhere is this situation worse than in Louisiana, where prosecutors seem to believe they are unconstrained by the Constitution.