Some states have similar rules to Oklahoma, allowing for certain methods if lethal injection is unavailable or found unconstitutional; others had acted like Utah, discontinuing a method but allowing inmates sentenced or found to have committed their crime before the method was dropped to select it.

You can see the full list of methods here.

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Other than lethal injection, how have states killed people over the past few years?

Virginia used the electric chair to execute convicted murderer Robert Gleason Jr. in 2013 because Gleason chose that method over lethal injection. In 2010, the state executed Paul Powell, convicted of raping one woman and murdering another, using the electric chair. That same year, Utah executed Ronnie Lee Gardner using a firing squad. Those executions are the exceptions, as every other execution since 2010 was carried out with lethal injection.

Here’s a helpful animation showing you just how states killed inmates over the years.

But haven’t there been a lot of mistakes made over the years and convictions eventually overturned?

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The Death Penalty Information Center says that between 1973 and this year, 144 people have been exonerated. (A recent study published in the Proceedings of the National Academy of Sciences found that at least 4.1 percent of people on death row could be wrongly convicted. And the authors note that “this is a conservative estimate,” so there could be quite a few additional people who were wrongly sentenced to death.

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How does the death-row population break down by race?

The current death row population is 41.7 percent black, 43.1 percent white and 12.6 percent Latino, according to the Death Penalty Information Center. Since 1976, 34 percent of all inmates executed were black (while 56 percent were white).

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But in cases where someone was executed after being convicted of a murder, the victims were far, far more likely to be white (77 percent) than black (15 percent). “The influence of race on the death penalty is pervasive and corrosive,” Richard Dieter, head of the Death Penalty Information Center, wrote in 1998.

Since lethal injection is the main way the country executes people, has the Supreme Court weighed in?

The Supreme Court has taken up the subject of lethal injection only once. In 2008 it decided by a 7 to 2 vote that the most commonly used method did not violate the Eighth Amendment’s ban on cruel and unusual punishment.

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But the lopsided decision in Baze v. Rees was misleading. Only two justices joined Chief Justice John G. Roberts Jr. in ruling that Kentucky’s protocol, which had been used only once, passed muster.

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“Simply because an execution method may result in pain, either by accident or as an inescapable consequence of death, does not establish the sort of ‘objectively intolerable risk of harm’ that qualifies as cruel and unusual,” Roberts wrote.

The plurality said judges considering challenges to lethal injection methods should decide whether they pose a “substantial risk of serious harm,” rather than the “unnecessary risk” proposed by lawyers for two men on Kentucky’s death row. Justices Antonin Scalia and Clarence Thomas agreed with the outcome, but said methods of execution should be deemed unconstitutional only if they were “deliberately designed to inflect pain.”

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Justices Ruth Bader Ginsburg and David H. Souter dissented. Ginsburg said execution methods must not create an “untoward, readily avoidable risk of inflicting severe and unnecessary pain” and she could not be sure Kentucky had taken the necessary safeguards.

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The decision ended what had become a de facto stay on executions while the case was being considered. But the splintered ruling led to even more legal challenges.

How many states have banned capital punishment?

There are 18 states that have officially abolished it. But there has been considerable movement in the recent past: Six of those 18 states abolished it since 2007, a pretty massive shift. (As many states outlawed capital punishment over that seven-year period as had outlawed it during the half-century preceding it.)

When was the last time a state outlawed the death penalty?

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Maryland abolished it in 2013. New Hampshire recently came very, very close to becoming the 19th state to ban the practice, but the repeal failed by a single vote (the state, the last in New England to maintain the death penalty, has come very close to banning it twice recently, once in 2000 and again in 2009). Meanwhile, Washington state suspended the death penalty earlier this year without officially banning it.

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Is the death penalty being used more or less these days?

It is being used much, much less. An average of 71.1 executions were carried out each year between 1997 and 2005; between 2006 and 2013, that number dropped to 44.3 executions per year. The number of executions peaked around the turn of the millennium and has declined considerably since that point:

How popular is the death penalty?

The death penalty is favored by a majority of Americans, but that number has plummeted. In 1996, 78 percent of Americans were in favor of capital punishment and 18 percent opposed it; by 2013, support had dropped to 55 percent while opposition had risen to 37 percent. The gap between supporters and opponents shrunk by 42 percentage points, which is a very big swing.

Americans do prefer lethal injection to other methods, though it’s unclear how the botched Oklahoma execution will impact that.

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How was the botched Oklahoma execution supposed to be carried out?

The inmates — and there were two executions scheduled for Tuesday night, one involving Clayton Lockett (who died from a heart attack 43 minutes after the first injection), the other involving Charles Warner (his execution has now been delayed) — were supposed to receive a trio of drugs through two intravenous lines inserted into each arm. These drugs included midazolam, vecuronium bromide and potassium chloride. A total of three executioners were to each inject one of the three drugs, according to the Oklahoma Department of Corrections.

Okay, so what about this drug shortage I keep reading about? Is it involved?

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This drug shortage, among other things, has caused major problems recently.

Until fairly recently, most executions in the United States relied on a three-drug combination that used an anesthetic, a paralytic drug and a drug that stopped the heart. In 2009, for instance, 50 of the 52 people executed were killed with a three-drug combination that used the anesthetic sodium thiopental. But not long after, Hospira, the sole manufacturer of the drug, said it wanted to get out of the sodium thiopental market due to its use in capital punishment.

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As a result, states began using pentobarbital, which was commonly used in executions since 2011. Again, the company that supplied the drug (Lundbeck, a Danish company) protested its use in executions. (In both of these cases, European companies or officials balked at the use of drugs manufactured in Europe — which is opposed to capital punishment — for executions.)

So states began scrambling to find other drugs and figure out other ways to carry out lethal injections. As a result, what had been a reasonably settled procedure became fraught with what experts called experimentation; states were trying out two-drug combinations, using just one drug or trying new combinations. (Oklahoma, for instance, had executed 17 people between 2010 and 2013; every single execution was carried out with a three-drug combination using either sodium thiopental or pentobarbital.) Due to the uncertainty, some states had even mulled turning back to options like the firing squad (Wyoming), the gas chamber (Missouri) and the electric chair (Virginia).

What does that mean for the Supreme Court’s ruling?

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The exact protocol approved by the Supreme Court has been discontinued because of the unavailability of the drugs. Deborah W. Denno, a professor at Fordham University and an expert on the death penalty, recently published a study of more than 300 cases citing Baze from 2008-2013 showing that “states’ lethal injection protocols have become increasingly diverse from one another, and from the original protocol evaluated by the Baze Court. Consequently, Baze has been rendered largely irrelevant a mere five years after its issuance.”

According to Richard Dieter of the Death Penalty Information Center, five states and the federal government have put executions on hold while designing new lethal injection methods.

Will the Supreme Court consider lethal injection again?

The Supreme Court has turned down applications for execution stays from inmates objecting to the secrecy surrounding the drugs that will be used. But at least three justices have shown interest in the issue.

Justices Ginsburg, Sonia Sotomayor and Elena Kagan said they would have granted a stay request from Michael A. Taylor, who was executed earlier this year in Missouri. They referred to “reasons well stated” by Circuit Judge Kermit E. Bye of the U.S. Court of Appeals for the 8th Circuit.

“I once again fear Missouri elevates the ends over the means in its rush to execute Taylor,” Bye wrote, saying the state is “unable to execute death row inmate with an FDA-approved form of injectable pentobarbital and thus uses compounding pharmacies to replicate the drug.”

He added: “Missouri has again, at the eleventh hour, amended its procedure and again is using a shadow pharmacy hidden behind the hangman’s hood.”

But so far, it appears, the rest of the court is not ready to take up the subject. In April, the court turned down without comment two petitions in which inmates sought a constitutional right to know the manner in which they would be executed. A petition from Louisiana inmate Christopher Sepulvado asked whether due process “entitles a condemned inmate with timely notice of the method by which he will be executed.”

Louisiana officials responded that Sepulvado already had the information he needed: “Louisiana will either execute him with pentobarbital or, alternately, a combination of midazolam and hydromorphone. Thus, he knows what drugs will be used to execute him.”

Could the country ever get rid of the death penalty?

It doesn’t appear likely any time soon. Again, even though public support is fading, a majority of the country still favors capital punishment.

“The death penalty is so entrenched in this country,” Denno said. “It’s like pulling up a tree with huge, deep roots. That’s going to take some time, and that tree’s never going to want to let go.”

She said that for many Americans, the death penalty is a part of history as well as a part of the national identity. But more states are likely to eventually get rid of the death penalty, she said, either through votes (as New Hampshire almost did) or simply halting the practice (like in Washington state).