EXHIBIT A is the corpses. Or rather, the curious paucity of them: like the dog that didn’t bark in Sherlock Holmes, the bodies are increasingly failing to materialise. Only 28 prisoners have been executed in America in 2015, the lowest number since 1991. Next, consider the dwindling rate of death sentences—most striking in Texas, which accounts for more than a third of all executions since (after a hiatus) the Supreme Court reinstated the practice in 1976. A ghoulish web page lists the inmates admitted to Texas’s death row. Only two arrived in 2015, down from 11 the previous year.

There is circumstantial evidence, too: the political kind. Jeb Bush, a Republican presidential candidate—who, as governor of Florida, oversaw 21 executions—has acknowledged feeling “conflicted” about capital punishment. Hillary Clinton, the Democratic frontrunner, said she “would breathe a sigh of relief” if it were scrapped. Contrast that stance with her husband’s return to Arkansas, during his own campaign in 1992, for the controversial execution of a mentally impaired murderer. Bernie Sanders, Mrs Clinton’s main rival, is a confirmed abolitionist.

The proof is overwhelming: capital punishment is dying. Statistically and politically, it is already mortally wounded, even as it staggers through an indeterminate—but probably brief—swansong. Fairly soon, someone will be the last person to be executed in America. The reasons for this decline themselves form a suspenseful tale of locked-room intrigue, unexpected twists and unusual suspects. So, whodunnit? Who killed the death penalty?

Twelve less angry men

Where politicians follow, voters often lead. Capital punishment is no longer a litmus test of political machismo because public enthusiasm for it is waning. Most Americans still favour retaining it, but that majority is narrowing. And one critical constituency—the mystery’s first prime suspect—is especially sceptical: juries.

Take the case of Eric Mickelson. In 2011 a jury in Louisiana sentenced him to death for murdering and dismembering an elderly man. Problems with the original trial led to a rerun this year: the new jury gave him life without the possibility of parole. According to a tally by the Death Penalty Information Centre (DPIC), a lobby group, overall only 49 people were sentenced to death in America in 2015, the lowest total in modern records. This despite the fact that, to serve in a capital trial, a juror has to be willing in principle to hand down a death sentence. (Actually doing so can be traumatic: Stewart Dotts “had always considered myself a reasonably tough guy”, but serving on a jury that passed a death sentence in New Jersey gave him many sleepless nights. “It’s an unfair burden to place on ordinary citizens,” Mr Dotts concludes.)

The widely available alternative of life without parole—which offers the certainty that a defendant can never be released—helps to explain that trend. So does the growing willingness of jurors, in their private deliberations, to weigh murderers’ backgrounds and mental illnesses; ditto the greater skill with which defence lawyers, generally better resourced and trained than in the past, muster that mitigating evidence. But the biggest reason, says Richard Dieter of the DPIC, is juries’ nervousness about imposing an irrevocable punishment. Behind that anxiety stands another, unwilling participant in the death-penalty story: the swelling, well-publicised cadre of death-row exonerees.

People like Harold Wilson, who served over 16 years for a ghastly triple homicide in Philadelphia before being exonerated in 2005. A decade later he is still fighting for compensation, as well as campaigning with Witness to Innocence, an exonerees’ organisation. He has “walked through hell”, Mr Wilson says. Ironically he thinks he might still be inside, doing life, if prosecutors hadn’t overreached in their quest to kill him. It’s a “broken-down system”, he believes. In 2015 alone, six more prisoners have been freed from death row.

Those mistakes implicate another suspect in the death penalty’s demise: prosecutors. The renegades who have botched capital cases—by suppressing evidence, rigging juries or concentrating on black defendants—have dragged it into disrepute. But some responsible prosecutors have also contributed, by declining to seek death in the first place. They have been abetted by another unlikely group: victims’ relatives.

Bethany Webb’s sister was among eight people killed in a Californian hair salon in 2011; her mother was shot, but survived. She wants the culprit to die “alone and unnoticed”, rather than being euthanised in an execution-night circus. The way prosecutors messed up the case—by needlessly deploying a jailhouse informant—has alerted her to the risks of injustices in others. Then there is the attritional legal rigmarole: the killer would smile at the victims’ families at court appearances, Ms Webb says; her mother is obliged to relive the trauma at each fresh hearing. A life sentence would have meant that “next time we see his face in the paper, it would be for his obituary”.

To avoid that protracted agony, says James Farren, district attorney of Randall County in Texas, “a healthy percentage” of families now ask prosecutors to eschew capital punishment. Mr Farren also fingers another key player in the death-penalty drama: the American taxpayer. Capital cases are “a huge drain on resources”, spiralling costs that—especially given juries’ growing reluctance to pass a death sentence anyway—have helped to change the calculus about when to pursue one, Mr Farren says. In 2011 a Californian study estimated that death-penalty trials cost the taxpayer an extra $1m a pop. Guilty verdicts mean lengthy and pricey appeals; death-row prisoners are often incarcerated in expensive isolation. Prosecutors are sometimes explicit about the trade-off between punishment and payment: in Arizona one withdrew his bid for a death sentence, court documents show, to help the county “meet its fiscal responsibilities”. Defence lawyers can be equally frank. Katherine Scardino says that, on being appointed in Texas, “the first thing I do is, I go start spending the state’s money”—on psychologists, investigators, the lavish cast of capital trials. Ms Scardino included an estimate of the cost of going to trial in a recent plea bargain.

The mystery of the empty vial

Even in vengeful Texas, she thinks, voters will eventually say of egregious villains, “Let him rot” in prison instead. Like exonerations, says Cassandra Stubbs of the American Civil Liberties Union, the exorbitant costs are a flaw that attracts widespread disapproval. They create an extra injustice: just as it was once unfair for death sentences to be reserved for the poorest criminals with the worst lawyers, so it is equally unjust for some to be spared on account of being tried in poor jurisdictions. A further upshot is an average delay between sentencing and executions that, at the last count, had risen to 16 years. The experience of Dale Cox, a prosecutor in Caddo Parish, Louisiana, is emblematic. He has been characterised as a juridical angel of death because of his outspoken advocacy of the ultimate punishment. Nobody prosecuted by Mr Cox has ever been executed.

Even when the appeals are exhausted, enacting a death sentence has become almost insuperably difficult—because of an outlandish cameo by the pharmaceutical industry. Obtaining small quantities of drugs for lethal injection, long the standard method, might seem an easy task in the world’s richest country; but export bans in Europe, American import rules and the decision by domestic firms to discontinue what were less-than-lucrative sales lines has strangled the supply. Arizona’s latest chemical misadventure is typical of the resulting travails. As Dale Baich, a public defender there, puts it, with several others the state was recently caught in “a drug deal gone bad”, after it tried to buy a deadly compound from a middleman in India; the batch was impounded by federal officials at Phoenix airport. This squeeze has obliged states to experiment with new concoctions and suppliers, not all of which are reputable. Those manoeuvres have given rise to gruesomely protracted executions—and still more litigation.

Lethal injection was intended to be reassuringly bloodless, almost medicinal (as, once, was electrocution). Should it become impractical, it is unclear whether Americans will stomach a reversion to gorier methods such as gassing and shooting: they are much less popular, according to polls. The death penalty’s coup de grace may come in the form of an empty vial.

Or it may be judicial rather than pharmaceutical: performed in the Supreme Court, the most obvious suspect of all. In an opinion issued in June, one of the left-leaning justices, Stephen Breyer, voiced his hunch that the death penalty’s time was up. He cited many longstanding failings: arbitrariness (its use varying widely by geography and defendants’ profiles); the delays; the questionable deterrent and retributive value; all those exonerations (Mr Breyer speculated that wrongful convictions were especially likely in capital cases, because of the pressure to solve them). He concluded that the system could be fair or purposeful, but not both. Meanwhile Antonin Scalia, a conservative justice, recently said he would not be surprised to see the court strike capital punishment down.

Cue much lawyerly soothsaying about that prospect. Yet the legal denouement is already in train: a joint enterprise between state courts, legislatures and governors. Of the 19 states to have repealed the death penalty, seven have done so in the past nine years. Others have imposed moratoriums, formal or de facto, including, in 2015, Arkansas, Ohio, Oklahoma, Montana and Pennsylvania. The number that execute people—six in 2015—is small, and shrinking. (After their legislature repealed the death penalty in May, Nebraskans will vote in 2016 on reinstating it; but their state hasn’t executed anyone since 1997.) These machinations may help to provoke a mortal blow from the Supreme Court. After all, the fewer states that apply the punishment, the more “unusual”, and therefore unconstitutional, it becomes.

Juries; exonerees; prosecutors, both incompetent and pragmatic; improving defence lawyers; stingy taxpayers; exhausted victims; media-savvy drugmakers: in the strange case of the death penalty, there is a superabundance of suspects. And, rather as in “Murder on the Orient Express”, in a way, they all did it. But in a deeper sense, all these are merely accomplices. In truth capital punishment is expiring because of its own contradictions. As decades of litigation attest—and as the rest of the Western world has resolved—killing prisoners is fundamentally inconsistent with the precepts of a law-governed, civilised society. In the final verdict, America’s death penalty has killed itself.