In California, nurses accused of serious wrongdoing have often been left free to practice for years while their cases were being investigated -- with patients unaware of the danger.

The board that oversees the state's registered nurses has taken more than three years, on average, to discipline nurses with histories of drug abuse, negligence, incompetence and violence. And even after completing investigations, it has put many nurses on probation and then failed to follow up when they ran into trouble again.

ProPublica's reporting on the issue led Gov. Arnold Schwarzenegger to replace most of the state nursing board, and its executive director resigned. State officials have begun to revamp some policies for investigations. But legislative efforts to overhaul the way California investigates and disciplines health workers have been blocked by strong opposition from labor unions.

And the problem is one that crosses borders: States fail to tell each other what they know, and a federal database of disciplinary action against health professionals is missing records in probably thousands of serious cases. Many nurses and other health workers who are disciplined in one state simply move to another and start with a clean license.