As Nebraska prepares to complete its first state-sanctioned execution in two decades, human rights groups are condemning the death penalty and denouncing officials for operating under a "cloak of secrecy" in obtaining the drugs that will be used to kill Carey Dean Moore—substances that have caused tens of thousands of accidental deaths across the U.S. in recent years.

Amnesty International denounced the state's move to participate in "the ultimate denial of human rights" after having carried out no executions since 1997.

"Rather than joining those states that have turned against this cruel and irrevocable punishment, Nebraska chooses to take a backward step that provides no constructive solutions to the challenges posed by violent crime," said Kristina Roth, Amnesty's senior program officer for criminal justice. "The U.S. capital justice system is fundamentally broken."

After a federal appeals court rejected a pharmaceutical company's attempt to block Moore's execution, the nearly four-decade inmate will be put to death using a never-before-used combination of the opioid fentanyl, valium, a muscle relaxant, and a substance that will stop his heart—which anti-death penalty advocate Sister Helen Prejean denounced as "human experimentation."

.@GovRicketts’s planned execution set for this morning is human experimentation. Carey Dean Moore will be pumped full of four different killing drugs. This drug combination has never been used in an execution before today. Nebraska officials won’t reveal who supplied the drugs. — Sister Helen Prejean (@helenprejean) August 14, 2018

The German drug maker Fresenius Kabi has sued Nebraskan officials, accusing them of of illegally obtaining the substances, which the state is rushing to use before they expire.

State prison director Scott Frakes wrote in an affidavit after the lawsuit was filed that "lethal substances used in a lethal injection execution are difficult, if nearly impossible, to obtain," detailing his attempts to secure drugs from at least 40 suppliers, according to the Washington Post. The state has not publicized how it obtained the substances.

Absolutely disgusting. Would that states with the death penalty spent half as much time developing policy to support people in their lives as they did figuring out ways to kill people. https://t.co/AwGLqnTQyZ — Melissa McEwan (@Shakestweetz) August 13, 2018

Fentanyl, which has never before been used in an execution in U.S., has moved to the center of the ongoing opioid crisis which has decimated communities across the country in recent years. The synthetic opioid is 25 to 50 times more powerful than heroin, and was involved in the overdose deaths of about 20,000 Americans in 2016 alone.

Earlier this month, Pope Francis announced that the Catholic Church would officially condemn state-sanctioned executions in all cases and work to abolish capital punishment around the world.

A growing number of states have done away with the death penalty in recent years, including Delaware, which abolished its use in 2016, and Maryland, which did the same in 2013.

"While more states are turning away from the death penalty, Nebraska officials are rushing to carry out an execution cloaked in secrecy with an untested four-drug scheme that carries immeasurable risks for unnecessary pain and a botched execution," said Danielle Conrad, director of the ACLU Nebraska.