ST. LOUIS -- A Missouri man convicted of killing a woman and her two children almost 20 years ago asked the U.S. Supreme Court to spare his life Monday, one day before his scheduled execution.

Mark Christeson, 37, is set for lethal injection this evening at the state prison in Bonne Terre.

Investigators said he raped and killed Susan Brouk and killed her 12-year-old daughter and 9-year-old son near their rural south-central Missouri home in 1998.

The nation's highest court halted Christeson's execution in 2014, just hours before it was scheduled.

Monday's appeal focuses on the same main issue that Christeson's attorneys cited then: His trial lawyers were so inept, they missed a 2005 deadline to file a federal court appeal, which is standard practice in death-penalty cases.

His lawyers also have argued Christeson has an IQ of 74 and was therefore mentally incapable of understanding his legal rights during his original trial.

Christeson's attorney, Jennifer Merrigan, declined comment when reached by phone Monday.

Maries County prosecutor Terry Daley Schwartze, who prosecuted the original case, called the killings "brutal."

"If there is a person who deserves the death penalty, it is this defendant," she said.

Christeson was 18 when he and his 17-year-old cousin, Jesse Carter, decided to run away from a home where they were living with a relative near Vichy, Missouri.

Armed with shotguns, they walked a half-mile to the neighboring home of Susan Brouk, planning to steal her Ford Bronco.

Once there, they used shoelaces to tie the hands of Brouk's 12-year-old daughter, Adrian, and her 9-year-old son, Kyle.

Investigators said Christeson forced Brouk into a bedroom and raped her.

The men later drove the family to a pond, where Brouk and her son were stabbed and thrown into the water to drown.

Adrian suffocated when Christeson pressed on her throat while his cousin held her.

The cousins fled to California, where they were caught eight days later.

Carter agreed to testify against Christeson and was sentenced to life in prison without parole.

Missouri executed 16 inmates from 2014 to 2015, second only to the 23 executions in Texas over the same two years.

Last year, Missouri had just one execution, largely because most of the 25 men on the state's death row have appeals remaining or they are unlikely to be executed because of medical or mental-health concerns.