Marcellus Williams, who was convicted of murder 21 years ago, has been executed in the Midwestern state of Missouri despite concerns raised over the integrity of the case.

The United States Supreme Court, the last body that could have overturned Williams’s death sentence, declined to intervene in the case on Tuesday.

Wesley Bell, whose office handled the original prosecution, had sought to block the execution due to concerns about the original trial.

“Even for those who disagree on the death penalty, when there is a shadow of a doubt of any defendant’s guilt, the irreversible punishment of execution should not be an option,” Bell said in a statement before the execution.