The painfully slow death of the death penalty in the United States continued to play itself out in 2015, reducing the ultimate punishment to levels not seen for the past 40 years, according to two new reports.

According to the annual report of the Death Penalty Information Center, capital punishment showed a sharp decline nationwide on all of the main indicators. The most jaw-dropping fall was in the number of new death sentences – a measure of the enthusiasm of the American people, as expressed through juries, for judicial killings – which reached a low not seen since the 1970s.

This year, 14 states and the federal government issued just 49 new death sentences. That’s a dramatic decline from the peak of 315 new sentences recorded in 1996 at the height of the moral panic around murder rates in big cities and the crack cocaine epidemic.

Given the protracted nature of death penalty litigation, many of those death row inmates condemned in the 1990s are only now coming close to being executed. But here, too, the statistics show a steep dropping-off.

This year, 28 executions have been performed, down from a 1999 peak of 98. Most tellingly, only six states were responsible for all those deaths, and within that rump an even smaller rump of just three states – Texas, Missouri and Georgia – performed the lion’s share.

Texas continues to be the nation’s capital for executions. It has carried out 13 this year – almost half the total for the country – followed by Missouri which continues its newfound passion for killing people with six executions, followed by Georgia with five.

Yet a closer look at Texas’s year shows that even at the core of the death penalty, the practice is withering. The annual report of the Texas Coalition to Abolish the Death Penalty records a steady decline.

At its peak in 1999, Texas condemned 48 people to death. This year that number had fallen to three.

The picture is even more dramatic if you zoom in to Harris County, which covers the city of Houston. In modern times it has been the epicenter of America’s judicial bloodlust, handing out a staggering 294 death sentences and executing more people than any other county in the nation.

Yet this year the number of new death sentences coming out of Harris County was precisely zero. As further evidence of the sea-change in Houston, a recent survey by the Kinder Institute for Urban Research at Rice University found that only 28% of the city’s residents preferred the death penalty over life without parole for first-degree murder.

A similar pattern can be found in the two other states that are still killing prisoners in large numbers – Missouri and Georgia. Though between them they were responsible for more than a third of the total number of executions in the country this year, neither of them imposed any new death sentences.

“The use of the death penalty is becoming increasingly rare and increasingly isolated in the US,” said Robert Dunham, executive director of the Death Penalty Information Center. “These are not just annual blips in statistics, but reflect a broad change in attitudes.”

The reasons for the withering on the vine of the death penalty is a subject of much academic and political debate. Factors include the growing cost of putting a prisoner to death, which have been estimated at up to $3m for a single case up to execution; the ongoing fiasco surrounding the struggle of death penalty states to acquire lethal injection drugs; studies that have found racial bias in sentencing; and international opprobrium.

Another major factor behind the erosion of confidence in the death penalty are concerns about the accuracy of capital sentences and the risk that innocent people are killed on the gurney. That danger was neatly illustrated again this year with the exoneration of six death row prisoners of all charges – one prisoner each in Alabama, Arizona, Florida, Georgia, Mississippi and Texas. That brings to 156 the total number of inmates who have been exonerated and freed from death row since 1973.

As the practice fades at the state level, the billion-dollar question is whether the US supreme court will step into the fray and move to prohibit capital punishment on constitutional grounds. The last time the nation’s highest court did so was in 1972 in the Furman v Georgia ruling that put a stop to the death penalty on grounds that it was too erratically applied.

The US supreme court justices Stephen Breyer, right, and Ruth Bader Ginsburg, left, expressed their constitutional objection to the death penalty in a recent dissenting opinion. Photograph: Brendan Smialowski/AFP/Getty Images

That moratorium proved to be only temporary, however, and executions restarted in 1977 with the death by firing squad of Gary Gilmore in Utah.

In recent times the supreme court has shown an unwillingness to be drawn back into the debate around capital punishment. In June, the nine justices rejected a challenge to Oklahoma’s new lethal injection protocol, voting according to the familiar 5-4 conservative-to-liberal split.

Two of the liberal justices – Stephen Breyer and Ruth Bader Ginsburg – openly expressed their constitutional opposition to the practice in a dissenting opinion. Legal observers are watching closely to see whether their action will develop into a new push for a moratorium, or whether the supreme court will sit back and allow the US to remain one of the few countries in the world that kills its citizens, alongside Iran, North Korea, Saudi Arabia and Somalia.