CNN  — 

Missouri prison officials executed convicted murderer Russell Bucklew by lethal injection Tuesday night despite objections from activists who thought the method would lead to a gruesome death, CNN affiliate KFVS reported.

Bucklew, 51, was convicted of first-degree murder, kidnapping and first-degree burglary in 1997. He was pronounced dead at 6:23 p.m. CT (7:23 p.m. ET), KFVS said.

Former Cape Girardeau County Prosecutor Morley Swingle, who tried Bucklew, said the death “was very peaceful.”

A KFVS reporter who witnessed the execution said she could see Bucklew on a gurney with a white sheet up to his chest. Bucklew was looking away from the viewing room. She said he turned his head so that he was looking straight ahead and was twitching his feet. The he let out a sigh, and there was no more movement. The curtains were closed for a few minutes, she said, and reopened with him still on the gurney.

There were no visible complications, she said. Swingle said Bucklew “closed his eyes and went to sleep.”

Activists were concerned that Bucklew’s disease would cause a gruesome death when he was administered drugs at his execution. He had a rare blood vessel disorder called cavernous hemangioma. The disease can cause tumors in the head and regular bleeding from the mouth, nose, eyes and ears.

Advocates for calling off the execution were worried the lethal injection would cause prolonged suffocation and excruciating pain.

Russell Bucklew mugshot

“We mourn the unnecessary, unlawful execution of Russell Bucklew tonight,” said Cassandra Stubbs, director of the Capital Punishment Project at the American Civil Liberties Union. “Missouri carried out an execution that risked torture and violated international law, despite Mr. Bucklew’s remorse and exemplary prison record. The idea that killing this man – who was terminally ill and had been a model prisoner for 23 years – was worth the stain on our democracy and our humanity is a disgrace.”

Swingle, the former prosecutor, said Bucklew’s death was in stark contrast to the violent death of his victim, Michael Sanders, who was shot in front of his former girlfriend and four children.

Gov. Mike Parson turned down a clemency request, said his press office, without providing additional detail.

Bucklew’s attorneys had argued the state should have considered death by lethal gas as an alternative.

In April, the Supreme Court ruled against Bucklew in a 5-4 decision, which means plans for the lethal injection can proceed.

Justice Neil Gorsuch said the Eighth Amendment “does not demand the avoidance of all risk of pain” in carrying out executions.

He also said Bucklew had failed to “present any evidence” that the substitution of lethal gas would “significantly reduce his risk of pain,” Gorsuch said.

But the ACLU said executing Bucklew would violate the Constitution’s prohibition against cruel and unusual punishment.

“What makes (Bucklew’s) execution different is that he has a medical condition that would make it one of the most gruesome in US history,” the ACLU wrote.

Bucklew had successfully staved off his execution in the past.

In 2014, the Supreme Court granted Bucklew a stay of execution, pending appeals.

Dr. Joel Zivot had filed an affidavit in the case saying that Bucklew’s airway is so “severely compromised and obstructed,” especially when he’s lying flat, that it could easily be ruptured, raising the risk that Bucklew could choke or suffocate.

“If you touch it, it bleeds,” Zivot wrote of Bucklew’s airway.

But Gorsuch, the current Supreme Court justice who wrote the majority opinion against Bucklew’s case this year, said justice must be served for Bucklew’s victims.

“The people of Missouri, the surviving victims of Mr. Bucklew’s crimes, and others like them deserve better,” Gorsuch wrote.

Bucklew was convicted of fatally shooting his ex-girlfriend’s presumed new boyfriend, Sanders, and firing at Sanders’ son before kidnapping Stephanie Ray Pruitt.

After raping his ex-girlfriend, court documents state, Bucklew was involved in a gunfight in which he and a Missouri state trooper were injured.

CNN’s Ariane De Vogue, Dave Alsup and Vivian Kuo contributed to this report.