David Cole is a professor at Georgetown University Law Center .

When Herman Wallace died Oct. 4 at age 71, he had been a free man for about 2½ days. Before that, he spent 41 years in solitary confinement in the Louisiana State Penitentiary in Angola. Forty-one years. The U.N. Special Rapporteur on Torture has concluded that, because of the mental suffering it inflicts, solitary confinement imposed as punishment for even 15 days can constitute torture or cruel, inhuman or degrading treatment. Yet for 41 years, Wallace was confined to a 6-by-9-foot cell for 23 hours a day. The prison allowed him out for one hour each day, to walk or exercise on his own. For most of his life, Wallace was deprived of any daily human contact other than with prison guards. His 68-year-old co-defendant, Albert Woodfox, placed in solitary confinement at the same time, remains there to this day.

What possible threat could warrant such extraordinary treatment? Were Wallace and Woodfox the leaders of a terrorist cell or international drug cartel, at risk of issuing orders that could result in death or serious injury? Was Wallace a threat to others even as he was dying of liver cancer? No. Prison officials were required, every six months, to assess whether solitary confinement was still necessary. Every time, they offered only one reason: “nature of original reason for lockdown.” Not once was it suggested that either man posed any continuing physical danger or risk of escape.

The “original reason for lockdown” was that Wallace and Woodfox were convicted of fatally stabbing a prison guard while incarcerated in Angola in 1972. On its face, that is a legitimate basis for solitary confinement, at least initially. But Wallace ultimately was released because a judge ruled that his conviction was rendered unconstitutional by a discriminatory grand jury. And because that required a new trial, the judge did not need to address the many other constitutional errors alleged in the case, including the suppression of exculpatory evidence showing that the only witnesses who had implicated Wallace at trial — fellow prisoners — had been threatened and offered pardons and other favors by prison officials in exchange for their testimony.

No physical evidence linked Wallace or Woodfox to the crime. Fingerprints at the scene did not match those of the defendants, but the prison did not attempt to test the prints against any but a handful of other inmates. Wallace and Woodfox maintained that they were framed because they had started a Black Panther Party chapter at Angola and had objected to the prison’s brutal conditions. Angola’s own museum Web site acknowledges that it was known in the late 1960s as “the Bloodiest Prison in the South.” From 1972 through 1975, more than 270 inmates were stabbed by other inmates . Corruption and sexual exploitation were rampant, and armed inmates patrolled the halls. One guard boasted in a published interview that “I’ve got just about every finger broke on both hands from punching” prisoners. Wallace and Woodfox led the protests of these conditions, and when a prison guard was found dead, they were charged with the crime.

Even if Wallace and Woodfox were guilty, how is 41 years in solitary an appropriate response? Solitary confinement is not a permissible punishment for a crime. It is justified, if at all, for prison security reasons when an individual poses a danger to others that cannot otherwise be reasonably contained. It can understandably be used for brief periods as discipline for infractions. But for all practical purposes, Wallace and Woodfox were sentenced to prolonged solitary confinement by prison authorities, not a judge. Wallace was released for only the last 2½ days of his life. Woodfox, whose conviction has also been overturned, remains in solitary pending the state’s appeal.

These cases are unusual only in the duration of the two men’s confinement. Thousands of inmates across the United States have been relegated to similar conditions in “super max” prisons — often for many years. In May 2012, the Center for Constitutional Rights filed a class-action lawsuit challenging prolonged solitary confinement at California’s Pelican Bay State Prison. More than 500 prisoners there have been in solitary confinement for longer than 10 years; more than 75 have been in solitary for longer than 20 years. Many are sent to solitary on little more than a suspicion of gang affiliation, which can be established merely by waving hello to a prisoner suspected of gang ties, possessing “gang-related” art or having the wrong tattoo. About the only way to get out is to “debrief” — to name names and implicate others, who are in turn likely to be sent to solitary.

Herman Wallace and Albert Woodfox may or may not have killed a prison guard in 1972. We will probably never know for sure. But one thing is certain: Their 41-year confinement to cells not much larger than a grave, without human contact, was cruel and inhumane treatment. Prolonged solitary confinement can be justified only in the most extraordinary circumstances. Yet it has become an all-too-routine part of the American system of mass incarceration.