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, 61, was scheduled to die by injection Tuesday evening at the state prison in Bonne Terre, about 50 miles south of St. Louis. It would be the seventh U.S. execution this year.

Johnson’s attorney, Jeremy Weis, said executing Johnson would violate the Eighth Amendment to the U.S. Constitution, which prohibits executing intellectually disabled people. On Monday, he asked the U.S. Supreme Court for a stay of execution.

“This is not a close case — Mr. Johnson is intellectually disabled,” the court filing stated.

The Missouri Supreme Court in August, and again on Friday, refused to step in despite Johnson’s history of scoring extremely low on IQ tests, dating back to childhood. Weis said Johnson also was born with fetal alcohol syndrome and lost about one-fifth of his brain tissue when a benign tumor was removed in 2008.

Republican Gov. Michael Parson on Monday declined to grant clemency despite the urging of several people, including the pope. A representative for Pope Francis wrote in a letter to Parson last week that the pope “wishes to place before you the simple fact of Mr. Johnson’s humanity and the sacredness of all human life.” Parson announced Monday he would not intervene.

It wasn’t the first time a pope has sought to intervene in a Missouri execution. In 1999, during his visit to St. Louis, Pope John Paul II persuaded Democratic Gov. Mel Carnahan to grant clemency to Darrell Mease, weeks before Mease was to be put to death for a triple killing. Carnahan, who died in 2000, was a Baptist, as is Parson.

In 2018, Pope Francis said capital punishment can never be sanctioned because it constitutes an “attack” on human dignity. Catholic leaders have been outspoken opponents of the death penalty in many states.

Racial justice activists and two Missouri congressional members — Democratic U.S. Reps. Cori Bush of St. Louis and Emmanuel Cleaver of Kansas City —have also spoke out in support of Johnson, who is Black. Bush planned to attend a prayer vigil near the prison on Tuesday.

Johnson’s crime shook Columbia nearly 28 years ago.

Johnson was a frequent customer of a Casey’s General Store. Court records show that on Feb. 12, 1994, he borrowed a .25-caliber pistol from his girlfriend’s 18-year-old son, with plans to rob the store for money to buy drugs.

In a 2004 videotaped interview with a psychologist shown in court, Johnson said he was under the influence of cocaine as he waited for the last customer to leave the store at closing time. Three workers were in the store: Manager Mary Bratcher, 46, and employees Mabel Scruggs, 57, and Fred Jones, 58.

On the video, Johnson said he became angry when Bratcher, who claimed not to have a safe key, tried to flush it down the toilet. He shot the victims with the borrowed gun, then attacked them with a claw hammer. Bratcher also was stabbed in the hand with a screwdriver. Police found two victims in the store’s bathroom, and the third in a cooler.

“This was a hideous crime,” said Kevin Crane, the Boone County prosecutor at the time. “It was traumatic, and it was intense.”

Police officers found a bloody screwdriver, gloves, jeans and a brown jacket in a nearby field and questioned Johnson within hours of the killings. At Johnson’s girlfriend’s house, officers found a bag with $443, coin wrappers, partially burned checks and tennis shoes matching bloody shoeprints from inside the store.

Johnson had previously asked that his execution be carried out by firing squad. His lawyers argued that Missouri’s lethal injection drug, pentobarbital, could trigger seizures due to the loss of brain tissue from the 2008 surgery.

Missouri law does not authorize execution by firing squad.

Johnson was sentenced to death in his first trial and two other times. The second death sentence, in 2003, came after the U.S. Supreme Court ruled that executing the mentally ill was unconstitutionally cruel. The Missouri Supreme Court tossed that second death sentence, and Johnson was sentenced a third time in 2006.

Of the six previous U.S. executions this year, three were in Texas and three involved federal prisoners.

The peak year for modern executions was 1999, when there were 98 across the U.S. That number has gradually declined and just 17 people were executed last year — 10 involving federal prisoners, three in Texas and one each in Georgia, Tennessee, Alabama and Missouri, according to a database compiled by the Death Penalty Information Center.