KILLER INSTINCTS.

Many American courthouses have a Kenneth Peasley. For years, he was the most feared prosecutor in Arizona's Pima County, which includes Tucson. He was widely known as the government lawyer who wouldn't plea-bargain, who left his adversaries seething, and who almost always won. When defense lawyers got together, they would talk about how Peasley had stuck his finger in their clients' faces, or how he wouldn't greet them in the hallway. "The defense lawyers hated him," Howard Hantman, a Pima County Superior Court judge, said. "But I always thought that was because he was so good. Watching Ken was like watching great theatre. He had an instinct for the jugular like no prosecutor I ever saw."

Peasley was more than just a local phenomenon. From 1978 until last year, he tried more than two hundred felony cases, including a hundred and forty homicides, and handled about sixty capital cases. He gave lectures around the country about how to try murder cases, and he won national awards. Steve Neely, who, as the county attorney, was Peasley's boss for eighteen years, said, "He was absolutely the most effective prosecutorial performer that I have ever seen or heard of." Peasley, a two-time state prosecutor of the year, is personally responsible for a tenth of the prisoners on Arizona's death row.

Last year, Peasley acquired another distinction: he was disbarred for intentionally presenting false evidence in death-penalty cases--something that had never before happened to an American prosecutor. In a 1992 triple-murder case, Peasley introduced testimony that he knew to be false; three men were convicted and sentenced to die. Peasley was convinced that the three were guilty, but he also believed that the evidence needed a push.

During the years of Peasley's rise and fall, the exoneration of prisoners on America's death rows has become increasingly common. According to the Death Penalty Information Center, since the mid-nineteen-seventies a hundred and seventeen death-row inmates have been released. Defense lawyers, often relying on DNA testing, have shown repeatedly how shoddy crime-lab work, lying informants, and mistaken eyewitness identifications, among other factors, led to unjust convictions. But DNA tests don't reveal how innocent people come to be prosecuted in the first place. The career of Kenneth Peasley does.

Although new evidence suggests that the co-defendants may have had nothing to do with the crime for which they were convicted, Peasley still believes that he prosecuted the right men. "I have never seen a case where I believed the prosecutors set out to prosecute someone whom they believed to be innocent," says Rob Warden, the director of the Center on Wrongful Convictions at the Northwestern University School of Law, whose staff members were involved in eleven of the eighteen recent exonerations on Illinois's death row. "They just get wedded to a theory and then ignore the evidence that doesn't fit." According to Barry Scheck, who co-founded the Innocence Project, which has won exonerations for more than a hundred and fifty convicted defendants, "After a while, some veteran prosecutors think that they can just trust their gut. Once you get to the point where you believe your instincts must be right, you quickly get to the point where you just deep-six inconvenient evidence."

One of the men Peasley prosecuted in the 1992 case is still on Arizona's death row. Unless a court intervenes, that man, Martin Soto-Fong, who was a seventeen-year-old high-school dropout at the time of the murders, will be executed, although no date has been set. The case already ranks as an extreme example of prosecutorial misconduct, but if Martin Soto-Fong is killed for a crime he didn't commit, it will stand for something far worse.

Shortly after ten o'clock on the night of June 24, 1992, in Tucson, an anonymous caller dialled 911 and said, "Yeah, I just walked into the El Grande. It's on Thirty-sixth, and, uh, there are two, uh, guys that work. . . . They laying down on the floor, and one's laying in a pool of blood, and there's no one in the store."