The botched execution in Oklahoma may have resulted in “cruel, inhuman and degrading treatment” according to international human rights laws, Rupert Colville, a spokesman for the United Nations High Commissioner for Human Rights, said Friday in Geneva.

Witnesses said that Clayton Lockett appeared to be in pain during his execution Tuesday night in Oklahoma, grimacing and clenching his teeth. Lockett was convicted of murder and numerous other charges after he and accomplices attacked and sexually assaulted two teenage women, one of whom the other victims said Lockett shot twice before she was buried alive.

His execution was called off after the vein used to insert the drugs collapsed and he died of a heart attack shortly afterward.

This episode was also the second case this year where “apparent extreme suffering” stemmed from a lethal injection in the U.S., Colville said, adding that this only reinforces the argument that the U.S. should abolish capital punishment. The United Nations opposes the death penalty.

Colville cited the execution of inmate Dennis McGuire in Ohio earlier this year. McGuire — convicted of raping and murdering a pregnant woman — spent several minutes snorting and gasping for air during his execution, which lasted for nearly half an hour.

States have been struggling to find the drugs to carry out lethal injections, with two key shortages stemming from objections made by European companies or officials.