AN INNOCENT MAN?….Discussions over the death penalty can include a variety of compelling angles, but one argument that’s tough get around is the fact that it’s one of the only forms of punishment that can’t be undone. It’s something to keep in mind when considering what happened in Texas to Ruben Cantu.

Texas executed its fifth teenage offender at 22 minutes after midnight on Aug. 24, 1993, after his last request for bubble gum had been refused and his final claim of innocence had been forever silenced.

Ruben Cantu, 17 at the time of his crime, had no previous convictions, but a San Antonio prosecutor had branded him a violent thief, gang member and murderer who ruthlessly shot one victim nine times with a rifle before emptying at least nine more rounds into the only eyewitness — a man who barely survived to testify.

Four days after a Bexar County jury delivered its verdict, Cantu wrote this letter to the residents of San Antonio: “My name is Ruben M. Cantu and I am only 18 years old. I got to the 9th grade and I have been framed in a capital murder case.”

A dozen years after his execution, a Houston Chronicle investigation suggests that Cantu, a former special-ed student who grew up in a tough neighborhood on the south side of San Antonio, was likely telling the truth.

The story chronicles what appears to be a criminal-justice tragedy. For supporters of the death penalty who insist that there are no documented incidents of an innocent person being executed, the Cantu example should, at a minimum, give them pause.

In this case, Cantu’s co-defendant, David Garza, who’d been reluctant to talk about the murder-robbery since his trial, has now signed a sworn affidavit saying he allowed Cantu to be falsely accused. What’s more, the man who survived the shooting — the only eyewitness — has recanted and told the Chronicle that he felt pressured by police to finger Cantu as the killer.

It gets worse.

[K]ey players in Cantu’s death — including the judge, prosecutor, head juror and defense attorney — now acknowledge that his conviction seems to have been built on omissions and lies.

“We did the best we could with the information we had, but with a little extra work, a little extra effort, maybe we’d have gotten the right information,” said Miriam Ward, forewoman of the jury that convicted Cantu. “The bottom line is, an innocent person was put to death for it. We all have our finger in that.”

Sam Millsap Jr., the former Bexar County district attorney who made the decision to charge Cantu with capital murder, says he never should have sought the death penalty in a case based on the testimony of an eyewitness who identified Cantu only after police officers showed him Cantu’s photo three separate times. […]

The Chronicle found other problems with Cantu’s case as well. Police reports have unexplained omissions and irregularities. Witnesses who could have provided an alibi for Cantu that night were never interviewed. And no physical evidence — not even a fingerprint or a bullet — tied Cantu to the crime.

Worse, some think Cantu’s arrest was instigated by police officers because Cantu shot and wounded an off-duty officer during an unrelated bar fight. That case against Cantu was dropped in part because officers overreacted and apparently tainted the evidence, according to records and interviews.

Before anyone jumps to accuse Bush of shirking his gubernatorial duties, his penchant for executions in Texas doesn’t apply in this case — Cantu’s death sentence was carried out a year before Bush became governor.

Politics aside, the Cantu execution offers a tragic example of a system gone terribly awry. Any discussion of capital-punishment moratoriums should start here.

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Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.