Ana Estevez says that a few days after she dropped her 5-year-old son off with his father, Aramazd Andressian Sr., for a week-long visit in April 2017, she spoke with her son via Skype, per court order.

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“I told him that I loved him more than anything and that he was my world, and I couldn’t wait to see him on Saturday,” she recalls of their conversation.

She says she had no idea that would be the last time she would speak with or see her son.

WATCH: Woman Claims Ex-Husband Who Killed Their Son Made The Child Lie During Custody Battle

Estevez and Andressian were involved in a bitter custody battle over their son, Aramazd “Piqui” Andressian Jr. She says she was so "paranoid" about Andressian Sr.’s behavior, she videotaped every custody exchange on her cell phone.

“He would just lie about everything, and so I videotaped that last time we exchanged, and I did not know that was the last time I would see him,” she tells Dr. Phil in an exclusive interview airing Tuesday.

WATCH: Mom On Learning Ex-Husband Was Arrested On Suspicion Of Murdering Their Son: ‘My Heart Just Broke Into A Million Pieces’

When Andressian Sr. didn’t show up with their child for the exchange at the end of the week-long visit, Estevez says she grew concerned and called the police. Andressian Sr. was finally located passed out in a South Pasadena park under the influence of prescription drugs, according to investigators – and Piqui was missing. Andressian Sr.'s car was doused with gasoline, and he claimed he didn’t know what happened or where Piqui was. Months later, Andressian Sr. admitted to murdering the young boy.

On Tuesday’s episode, hear more from Estevez about her relationship with Andressian Sr. And, hear how she is striving to keep her son’s memory alive – and how you can help. Check here to see where you can watch.

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Woman Describes How Her Ex-Husband Allegedly Murdered Their 5-Year-Old Son