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During the sentencing phase of the trial for the man convicted of torturing and killing his girlfriend’s young son Gabriel Fernandez in 2013, Gabriel’s father testified before the court on Tuesday.

Arnold Contreras, the Palmdale boy’s father, was in prison during the eight months Gabriel lived with his mother and her boyfriend, Isauro Aguirre — a living situation that ultimately led to his death.

Contreras spoke out for the first time on Tuesday, telling the court he feels guilty that he wasn’t there for his son.

He recalled feeling helpless and shattered when he learned of Gabriel’s death while serving time in Riverside County. To this day, he is plagued with thoughts of his son’s death and unable to sleep, he testified.

Gabriel had gone to live with his maternal grandparents while Contreras was in jail, but the father always opposed the idea of Gabriel being in the custody of his biological mother, Pearl Fernandez, and defendant Isauro Aguirre.

Earlier this month, Aguirre was found guilty of first-degree murder. The jury also found true a special circumstance allegation that Aguirre had inflicted torture on the 8-year-old.

During the time Gabriel lived with Fernandez and Aguirre he was brutally abused, subjected to inhumane conditions and ultimately died.

Fernandez is set to be tried on charges related to her son’s death once Aguirre’s proceedings are wrapped up.

The penalty phase of Aguirre’s trial, which began Monday, is expected to last up to ten days. The jury is weighing whether Aguirre should face the death penalty, the sentence prosecutors have requested.

Aguirre’s defense argues that the defendant acted out of rage and did not intentionally kill the boy.

Over the course of the week, several others, including nurses and forensic experts, are expected to testify against Aguirre.