An Oklahoma man has pleaded guilty in the strangling deaths of his ex-girlfriend and her four young children.

Joshua Durcho, 29, avoided the death penalty by his guilty plea on five counts of first-degree murder, entered on Friday in El Reno, Oklahoma, for the brutal slayings of his 25-year-old former lover and her family. 

Durcho had strangled Rust and then killed her children because he feared they could testify against him. He then stacked the children's bodies in a bathtub partially filled with water in Rust's apartment.

Last month, Dr. Shawn Roberson, a forensic psychologist with the Oklahoma Department of Mental Health and Substance Abuse Services, testified that Durcho's mental functioning was 'at the low borderline range.' 

Nonetheless, a judge rejected claims that Durcho is mentally disabled and ineligible for the death penalty.

Oklahoma law bars death sentences if a defendant meets the state definition of mental retardation, which includes an IQ of 70 or below and 'significant limitations in adaptive functioning,' the real-life communications, self-care and work and social skills people need to live independently and function safely and appropriately. 

In addition, the onset of mental retardation must occur before the age of 18.

Roberson said Durcho has been administered four IQ tests since he was 11 years old and scored between 72 and 78 on the tests. 

The most recent tests were administered in 2009 and 2010, and Durcho scored 72 on both.