Anti-capital punishment advocates on Thursday condemned an announcement by Oklahoma Attorney General Mike Hunter this week that the state would begin using nitrogen gas to execute death row inmates, after being unable to secure lethal injection drugs.

Just reading about how much work has been put into finding a way to effectively kill human beings by the state of Oklahoma gives me the chills. As an Oklahoman, I am ashamed. - Oklahoma officials plan to use nitrogen for executions - ABC News - https://t.co/kUAYsBmCPj via @ABC — Sr. Theresa Aletheia (@pursuedbytruth) March 15, 2018

Following challenges by prisoners and rights advocates, some manufacturers have stopped selling certain kinds of drugs to states for lethal injections, leaving some ready to use those on death row as guinea pigs as they experiment with the ways to carry out executions.

Oklahoma temporarily suspended its use of the death penalty in 2015 after the state used the wrong drug in one execution and left another inmate writhing in pain for 43 minutes due to an improperly placed IV. Later, that inmate died from what was determined to be a massive heart attack.

Hunter equated halting executions to "sitting on the sidelines" in a statement on Wednesday and said using nitrogen gas to kill inmates would be "effective, simple to administer, [and] easy to obtain."

Critics, including Robert Dunham, executive director of the Death Penalty Information Center, raised concerns over the experimental nature of the newly proposed procedure.

"This is another execution process that is untested, untried, and experimental," he told the Independent.

According to the Independent, the American Veterinary Medical Association has deemed the use of nitrogen gas as unacceptable for euthanizing animals, and has said it would take several minutes for a 70-pound animal to die if the gas was administered.

Nitrogen gas has been used to euthanize people who have requested the procedure.

But "there is a significant difference between when someone is seeking to terminate their own life—and they are not struggling and are willfully participating—and what happens when you involuntarily seek to terminate the life of someone who wants to live, and who will be struggling and trying not to breathe," Dunham told the New York Times.

On social media, anti-death penalty activist Sister Helen Prejean denounced Oklahoma's plan to asphyxiate inmates.

After three years without an execution, Oklahoma announced today that it is moving towards reopening its death chamber for business, using asphyxiation via inert gas as its method of killing. We must fight this tooth and nail! 17 men are lined up for the state's death chamber. — Sister Helen Prejean (@helenprejean) March 14, 2018

The suggestion by authorities that this new method is "more humane" ignores the fact that it is an untried method, human experimentation practiced by the state upon its citizens. — Sister Helen Prejean (@helenprejean) March 14, 2018