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 on Monday, a day before his scheduled execution.

Mark Christeson, 37, is set for lethal injection Tuesday evening at the state prison in Bonne Terre. Investigators said he raped and killed Susan Brouk, and killed her 12-year-old daughter and her 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 that they missed a 2005 deadline to file a federal court appeal, which is standard practice in death penalty cases.

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