A prisoner who spent 20 years on death row in the US has been executed despite prosecutors and the victim’s family saying he should not die.

Marcellus Williams, 55, was put to death by lethal injection shortly after 6 p.m. local time on Tuesday at a prison in Bonne Terre, in northern Missouri, state officials said.

Hours earlier, the U.S. Supreme Court had declined to intervene in the case, as had the state’s governor and the Supreme Court the day before.

Williams, whose son watched him die from another room, was found guilty in 2003 of the murder of Felicia “Lisha” Gayle, who was stabbed to death five years earlier during a burglary at her St. Louis home.

According to Wesley Bell of the St. Louis County District Attorney’s Office, which handled the original conviction, there were too many doubts about the case to sentence him to death.

Mr Bell raised concerns including the reliability of the two key witnesses in the trial, the exclusion of black jurors because of their race (Williams was also black) and the lack of the inmate’s DNA on the murder weapon after new testing.

Further investigation revealed that it contained DNA traces of a public prosecutor and a detective who worked on the case, as both had handled the knife without gloves.

In a written statement, Mr Bell said that “where there is even a shadow of doubt about innocence, the death penalty should never be an option”.

Last month, prosecutors and Williams’ legal team agreed to commute his sentence to life in prison, but the move was blocked by the Missouri Supreme Court at the request of Attorney General Andrew Bailey.

Earlier this month, a state judge upheld the conviction, concluding that the lack of evidence on the knife was not enough to prove his innocence.

The ruling was upheld by the state Supreme Court on Monday, the same day that Missouri Gov. Mike Parson, a Republican, also rejected a request for clemency.

Mr Parson said in a statement after Williams’ execution that he hoped it would bring “final closure” to a case “that has languished for decades and has victimised Ms Gayle’s family over and over again”.

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It was the third time Williams was sentenced to death, after being reprieved in 2015 and 2017.

In their request for clemency, Williams’ lawyers told the U.S. Supreme Court that Ms. Gayle’s family believed he should not be executed because there were doubts about his guilt, and they had approved the August plea agreement.

Williams’ public defenders said they could not understand why the “recognized racial discrimination” was not raised in his trial.

Williams, who converted to Islam, said in his handwritten closing statement: “All praise be to Allah in every situation!!!”.