John Thompson spent 18 years in a Louisiana prison, 14 of them in a windowless, six-by-nine-foot death row cell. According to a federal appeals court, "There were multiple mentally deranged prisoners near him who would yell and scream at all hours and throw human waste at the guards." Thompson, whose execution was scheduled half a dozen times, was a few weeks away from death by lethal injection when his life was saved by a bloody scrap of cloth.

Although four prosecutors in the Orleans Parish District Attorney's Office were aware of this evidence, Thompson didn't learn about it until 14 years after his death sentence, when an investigator hired by his pro bono lawyers discovered a crucial crime lab report. In a case the Supreme Court agreed to hear last week, the prosecutors' boss, former D.A. Harry Connick, argues that his office should not be held responsible for the egregious misconduct that led to Thompson's 18-year ordeal. But if it isn't, no one will be, an outcome that would not only deprive Thompson of compensation but endanger every American's due process rights.

In 1985 Thompson, then 22, was arrested for the murder of a hotel executive who was robbed and shot outside his New Orleans home. After Thompson's picture appeared in a local newspaper, three people who were victims of an armed robbery a few weeks after the murder thought they recognized their assailant.

Unbeknownst to Thompson or his attorney, the robber was cut while scuffling with one of his victims, leaving blood on the man's pants. The blood was type B; Thompson's blood type is O. He never got a chance to present this exculpatory evidence because his prosecutors never turned it over, even though they were constitutionally required to do so.

The prosecutors decided to try Thompson for the robbery first, hoping to prevent him from taking the stand at his murder trial (since that would allow them to impeach his credibility by mentioning the earlier conviction) and to enhance the likelihood of a death sentence. They succeeded on both counts.

After the concealed evidence came to light, Thompson's robbery conviction was thrown out, and he won a new murder trial, during which he presented 13 pieces of evidence that, like the blood test, had been withheld by prosecutors. The jury acquitted him after deliberating for half an hour.

In 2003 Thompson won a $14 million award from a federal jury that concluded D.A. Connick had acted with "deliberate indifference" by failing to train his underlings in their constitutional obligations. An appeals court upheld that award in 2008.

It's hard to say which would be more appalling: if prosecutors intentionally hid the blood test, or if they did not know they were required to share it. There is evidence to support both theories.

In 1994, after he was diagnosed with terminal cancer, one prosecutor told a former colleague (who kept this information to himself for five years) that he had deliberately withheld the blood evidence. But it's not clear he knew that was illegal at the time of Thompson's robbery trial, and testimony in Thompson's civil case indicated that other prosecutors in the D.A.'s office, including Connick himself, did not know they were legally required to share the crime lab report. Thompson's lawyers note that one prosecutor's "misunderstanding" of the relevant Supreme Court ruling "was so fundamental that the district judge visibly registered surprise," prompting him to change his testimony.

The Supreme Court has ruled that local governments can be held liable for failing to train officials in their constitutional responsibilities when the need is "obvious," as with teaching police officers the proper use of deadly force. The need for prosecutors to respect defendants' due process rights is no less obvious. And since prosecutors themselves have absolute immunity for their trial-related misconduct, the threat of lawsuits against their employers is an important safeguard to prevent the pursuit of victory from trumping the pursuit of justice.

Jacob Sullum is a senior editor at Reason and a nationally syndicated columnist.

© Copyright 2010 by Creators Syndicate Inc.