Alabama killed Kenneth Smith last night in the nation’s first known nitrogen gas execution.

“Tonight Alabama caused humanity to take a step backward. I’m leaving with love, peace and light. Thank you for supporting me. Love all of you,” were Smith’s final words.

Smith was strapped to a gurney in the execution chamber with a “full facepiece supplied air respirator” affixed to both his face and the gurney before the warden activated the nitrogen gas, slowly suffocating him.

Smith’s attorneys fought against the use of nitrogen and the state’s protocol for administering it in the months leading up to Smith’s execution, but lost a final appeal to the Supreme Court on Thursday evening.

Anti-death penalty advocates and international officials also protested the execution over ethical concerns. United Nations experts had expressed “alarm” at the state’s decision to use an untested method, noting it could amount to torture or cruel, inhuman or degrading treatment banned under international law.

On Thursday, a researcher from Amnesty International USA condemned Alabama for its decision and called on state officials to issue a death penalty moratorium.

“The death penalty is the ultimate cruel, inhuman and degrading punishment and accounts of Kenneth Smith’s last moments simply show that there is no humane way to take someone’s life. It is high time for those in power to stop trying to fix the failed experiment that is the death penalty,” Justin Mazzola said in a statement.

Speculation over Smith’s execution came not long after Smith was pronounced dead.

One reporter said Smith “appeared to shake and writhe on the gurney for at least two minutes at the start of the execution,” before asking Alabama Department of Corrections Commissioner John Q. Hamm whether that was “expected” or an indication of “suffering” during a news conference following the execution. Lawyers for the state were clear that Smith would become unconscious within seconds of the gas being administered, according to court documents.

Smith’s spiritual advisor Rev. Jeff Hood was with Smith Thursday evening blessing him repeatedly and gesturing the sign of the cross. Hood painted a troubling picture of the execution for reporters during a press conference that followed. Members of Death Penalty Action Network and L’Chaim! Jews Against the Death Penalty joined him.

“A great evil was unleashed on the state of Alabama tonight and we have got to do whatever it takes to make sure that this never ever happens again,” Hood said to reporters. “I hope this is a moment in which the people of Alabama say ‘Not in my name.’”

Smith was executed for his role in the 1988 murder of Elizabeth Sennett. Sennett’s family made a statement Thursday evening. Her son, Mike Sennett, said the family had long forgiven the people involved in his mother’s murder. “Nothing that happened here today is going to bring mom back,” he said. “It’s kind of a bittersweet day. We’re not going to be jumping around whooping and hollering… but we’re glad this day is over.”

Alabama was the last of three states to authorize nitrogen hypoxia as an execution method, though the others — Mississippi and Oklahoma — have yet to use it.

Alabama Attorney General Steve Marshall said the state achieved something “historic,” shortly after Thursday’s execution and criticized anti-death penalty advocates who questioned whether nitrogen hypoxia was humane.

“They don’t care that Alabama’s new method is humane and effective, because they know it is also easy to carry out.”