The Exonerator

Craig Watkins may be the only prosecutor in America who is making his name getting people out of prison.

As district attorney of Dallas County, Mr. Watkins is using DNA evidence to investigate more than 400 guilty verdicts notched up by his predecessors. His office's Conviction Integrity Unit, launched last year for this purpose, has so far cleared six men wrongly convicted of rape, murder or robbery.

In the past two decades, more than 200 convicts nationwide have been freed thanks in part to DNA testing. The tests involve taking biological material such as blood from the person convicted and comparing it to a sample left at the crime scene. These efforts are usually spearheaded by defense lawyers, not prosecutors.

Mr. Watkins, who became the first African American district attorney in Texas when he was elected in 2006, said in a recent interview that he has been accused of being "a criminal-loving DA, a hug-a-thug DA." But he says such criticism of him and his office misses the point: "We have the constitutional obligation to seek justice."

Each exoneration has...