Ellen N. Biben, New York's inspector general, has the credentials of the experienced and independent prosecutor needed to investigate the Nassau County police crime-lab fiasco. In appointing her, Gov. Andrew M. Cuomo also provided a crucial tool to get the job done, the power of the subpoena.

Biben has made her reputation in racketeering cases and those involving fraud by public officials. She worked for a decade in the Manhattan district attorney's office and for the past four years with Cuomo when he was state attorney general, serving as his special deputy for public integrity. Just as important, Biben can get started as soon as yesterday. As inspector general, she has a built-in office and access to a seasoned staff.

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At the start, Biben needs to determine the scope of what went wrong with evidence tested in a crime lab that has been put on probation twice since 2006. Is the concern over evidence limited to the testing of illegal drugs or are ballistics, fingerprint and blood-alcohol samples also suspect? She needs to determine the consequences of the lab's shortcomings for past and current prosecutions. This doesn't mean that every conviction based on evidence tested by this lab will be undermined. It's a lengthy and difficult process to overturn a conviction, especially if the defendant pleaded guilty. But if there are individuals who were unfairly prosecuted, they should be identified as soon as possible.

Biben must follow the evidence wherever it leads, even if it takes her into the top ranks of the police department and beyond. Was the compromised lab work the result of shortcuts by those in cushy jobs who weren't properly trained or supervised? Is it negligence or criminal? Was there a cover-up after the lab was put on probation? Who was aware of the problem and when did they learn about it? Tell us who must be held accountable.

And, at the end, Biben should give us specific recommendations on how to stop this from happening again. Doing this well from start to finish is critical to the integrity of the criminal justice system, and the concern about its reputation extends beyond the borders of one county.

The governor stepped into the case late Friday, the day after State Supreme Court Justice William Donnino rejected an effort by Nassau District Attorney Kathleen Rice to prepackage the special prosecutor and limit the scope of his probe. Rice sought to name Eric Dinallo, an expert on securities and insurance law. Donnino dismissed Rice's request as wrong on the issue of who had the legal authority to appoint the special prosecutor. In addition, he said a lawyer in private practice was not the best choice, explaining, ". . . a public prosecutor is a person most directly accountable to the people for his or her fair investigation and prosecution, if necessary, of those who may be liable for misconduct."

Donnino got it right and so did Cuomo.