We may be there already, as I learned last week in Missouri. That state has been the focus of considerable death-penalty coverage because of Pope John Paul II’s moral appeal to Gov. Mel Carnahan for clemency for Albert Mease, who was scheduled to die during the papal visit to St. Louis in January. Carnahan, a pro-death-penalty Democrat who has presided over 27 executions in six years, granted clemency. The results are perverse. Mease, who confessed to a grisly triple murder, is alive; Roy Roberts, who asserted his innocence and passed a lie-detector test, was executed last week.

On the night before he died by lethal injection, I interviewed Roberts at the Potosi Correctional Center, a sparkling prison south of St. Louis. Roberts did not go gently, insisting until the end that he was framed. I don’t know for sure that Roberts is innocent, but his appeal for clemency raised plenty of reasonable doubt about his guilt. The problem is that with so many executions nowadays the American machinery of death is less equipped to recognize the doubt. And, in 1996, Congress sharply curtailed last-minute death-penalty appeals, clearing the way for more mistakes. Roberts may well have been one of the few, and his death shook me.

Roberts grew up in a poor white section of St. Louis, saved a friend from drowning at 17, served a little time for stealing CB radios and was convicted in 1979 of robbing a restaurant. He was a good prisoner–eventually earning two degrees–but in 1983, he took part in a drunken prison riot that led to the stabbing death of a guard, Thomas Jackson. Roberts was convicted of holding Jackson down while two others stabbed him in the eye and heart. The two stabbers are alive today–one got life; the other is awaiting a new trial. Their clothes were soaked with Jackson’s blood; Roberts’s clothes were clean, and he had no weapon. In other words, there was no physical evidence in the case, which rested on four detailed eyewitness accounts.

But there was a problem with those eyewitnesses. Their stories were based on what’s called “evolving” testimony. In the aftermath of the riot, the Missouri Department of Corrections issued a 17-page report that made no mention at all of “Hog” Roberts, who weighed 337 pounds at the time and stood out, in the memorable words of a guard, “like a red rose in the Sahara Desert.” The report’s author suggested hypnosis to jog one guard’s memory of who was involved. He still didn’t name Roberts. That guard also viewed Roberts in a lineup. Still no recognition of the big man. Then Corrections Officer Denver Halley, who had not identified Roberts in his initial report, suddenly fingered him. (Roberts told me Halley held a grudge against him because he quit working in the prison laundry.) By the time of trial, the other guards and an inmate trying to beat the rap developed specific memories of Roberts’s guilt that tracked with Halley’s.

The defendant’s court-appointed attorney did little to exploit the difference between their first recollections of the riot and what they testified to in court. The attorney did manage to elicit testimony from a guard that he was having a fist fight with Roberts on the other side of the room at the time of the murder. It wasn’t enough. By the time another man confessed last month to the restaurant robbery–the case that put Roberts in jail in the first place–it was much too late.

Missouri Attorney General Jay Nixon makes a spirited case for the prosecution. There was no blood on Roberts because he grabbed Jackson from the back. Roberts could have had a fist fight across the room and held Jackson down a few minutes before or after. Higher courts found no reversible errors in the trial or Roberts’s legal representation. All plausible if debatable points. But if Halley, the key witness, testified at the trial that he was absolutely sure it was Roberts who held down Jackson, why did he fail to identify Roberts not once, but twice in earlier reports? That’s the nub of the question, and beyond backing the jury and describing the general confusion of the melee, neither Nixon nor the governor’s chief counsel nor Halley himself could answer it. “It’s been too long ago,” Halley told me.

As for the polygraph exam, administered last month, Nixon says, “I just don’t believe in lie-detector tests, and this one was 15 years too late.” But as Roberts himself told me, “They never say that when they’re done by police.” In fact, studies show that polygraphs are, at worst, 70 percent reliable–not enough to be admissible as evidence, but certainly sufficient to raise doubts when a man’s life is at stake. Where was the Missouri media on that story? You’d think that keeping the government from killing people who might be innocent would be Job 1 for reporters. But they were mostly MIA.

Carnahan’s critics say that he denied clemency because he’s on the defensive for acceding to the pope’s request in January and he doesn’t want to look soft on the death penalty when he runs for the Senate against hard-liner John Ashcroft next year. I hope that’s not true. In the meantime, I’ll remember Roy Roberts as perhaps the unluckiest man I ever met. Was he executed for politically convenient reasons for a crime he didn’t commit while serving a sentence he didn’t deserve? We’ll never know for sure. Roberts blamed himself for “missed opportunities” in life. His death, in turn, should be an opportunity to take a harder look at the system, so we don’t have to wonder whether we’re making the ultimate mistake.