Under Oklahoma law, a murder defendant is ineligible for a death sentence if he has an IQ of 70 or below, plus significant limitations in two or more adaptive skills such as communication and self care.
Also, by law, the mental condition must have been present before the defendant's 18th birthday.
Durcho scored 72 twice on IQ tests given since his arrest in 2009, records show.
Prosecutors allege Durcho strangled Summer Rust, 25, when she picked up the phone to call police on him. Prosecutors say he had vowed he would never go back to prison.
They allege he strangled Rust's three daughters, Kirsten Rust, 7, Autumn Rust, 7, and Evynn Garas, 3, and her son, Teagin Rust, 4, to eliminate them as witnesses.
The victims' bodies were found Jan. 12, 2009, in the apartment. The children's bodies had been stacked in a bathtub partially filled with water. Durcho was arrested after a car wreck in Texas.