ATMORE, Ala. (NewsNation) — Spiritual adviser the Rev. Jeff Hood says Thursday’s execution of convicted murderer Kenneth Eugene Smith by nitrogen gas hypoxia in Alabama was “absolutely horrific.”

Hood was with Smith during the last moments of his life.

“Slowly, he (Smith) began to spit mucus, perhaps vomit, came out of his mouth, Hood said exclusively on “Banfield.” “As I was standing there, he convulsed for minutes. It was absolutely one of the most horrifying things I’ve ever seen.”

The execution came after a last-minute legal battle in which Smith’s attorneys contended the state was making him the test subject for an experimental execution method that could violate the constitutional ban on cruel and unusual punishment. Federal courts rejected Smith’s bid to block it, with the latest ruling coming Thursday night from the U.S. Supreme Court.

The state had predicted the nitrogen gas would cause unconsciousness within seconds and death within minutes. State Attorney General Steve Marshall said late Thursday that nitrogen gas “was intended to be — and has now proved to be — an effective and humane method of execution.”

When asked about Smith’s shaking and convulsing on the gurney, Alabama Department of Corrections Commissioner John Q. Hamm said they appeared to be involuntary movements.

Critics who had worried the new execution method would be cruel and experimental said Smith’s final moments Thursday night proved they were right. Marshall, however, characterized it Friday as a “textbook” execution.

Hood said everyone in the chamber during the 22-minute execution was “visibly surprised.”

“In the protocol, they told us he would be unconscious in seconds. We experienced 22 minutes of horror,” Hood said.

The Associated Press contributed to this report.