Last-minute court intervention on Tuesday was the last obstacle to the execution of Ernest Johnson, a Missouri man convicted of killing three convenience store workers during a closing-time robbery nearly 28 years ago.

Johnson was scheduled to die by injection Tuesday evening at the state prison in Bonne Terre. It would be just the seventh U.S. execution this year.

Johnson’s attorney, Jeremy Weis, says executing Johnson would violate the Eighth Amendment to the U.S. Constitution, which prohibits executing intellectually disabled people.

Johnson's attorneys on Monday asked the U.S. Supreme Court for a stay of execution, writing: “This is not a close case – Mr. Johnson is intellectually disabled."

“The state is prepared to deliver justice and carry out the lawful sentence Mr. Johnson received in accordance with the Missouri Supreme Court's order,” Governor Parson said on Monday.

"Mr. Johnson was tried and convicted for the brutal murder of three innocent victims during a robbery in 1994. The evidence showed Mr. Johnson went to great lengths to plan and conceal his crime. Three juries have reviewed Mr. Johnson's case and recommended a sentence of death. Mr. Johnson's claim that he is not competent to be executed has been reviewed and rejected by a jury and the courts six different times, including a unanimous decision by the Missouri Supreme Court. Mr. Johnson has received due process under the laws in the state trial court, Missouri Supreme Court, federal district court, 8th Circuit Court of Appeals, and the United States Supreme Court, Parson said in a statement.