Russell Bucklew is seen in a police mugshot released by the Missouri Department of Corrections on September 30, 2019. Image Credit: Reuters

Washington: A convicted murderer with a rare medical condition was executed Tuesday in Missouri despite his assertion that lethal injection would cause him terrible suffering.

Russell Bucklew - who was found guilty of killing his ex-girlfriend's new boyfriend in 1996, then abducting and raping her - was pronounced dead at 6:23 pm (2323 GMT).

Bucklew, 51, suffered from cavernous hemangioma, a blood vessel condition that hindered his breathing. For the past year, he had been using a tracheotomy tube.

His lawyers had said his execution could be horrific if done by lethal injection.

"His face is swollen by the bulging, blood-filled tumors in his face, head and throat," the lawyers wrote in a petition asking Missouri's governor to commute the sentence and have Bucklew serve a life term instead.

"These unstable tumors are highly likely to hemorrhage during the stress of the execution, causing Russell to cough and choke on his own blood."

Governor Mike Parson, a Republican who supports the death penalty, turned down the request Tuesday morning.

Bucklew was the 17th inmate put to death in the US this year.

"We mourn the unnecessary, unlawful execution of Russell Bucklew tonight," Cassandra Stubbs, the director of the Capital Punishment Project at the American Civil Liberties Union, said in a statement.

Authorities in Missouri had twice set dates for his execution, in 2014 and 2018.

Both times, the US Supreme Court ordered last minute stays.

Some former prison wardens came out in favor of Bucklew's request, saying they wanted to spare their colleagues the risk of a messy, botched execution.

"Participating in executions places a tremendous weight on the shoulders of the execution team," they said in a court brief.

"When as here, an execution is unlikely to go smoothly, and is likely to result in unnecessary pain and suffering, the burden of participation becomes unbearable."

In April, the US Supreme Court rejected arguments over how cruel it would be to execute Bucklew by lethal injection.

"The Eighth Amendment does not guarantee a prisoner a painless death," the majority judges said in a 5-4 ruling.

The amendment outlaws "cruel and unusual punishments."

The court's four liberal judges disagreed.

"Executing Bucklew by lethal injection risks subjecting him to constitutionally impermissible suffering," Justice Stephen Breyer wrote on behalf of the liberals.