According to prosecutors, Mercado barged into his ex-girlfriend Serena Tarin's home by shooting off the lock on the door. He then went from room to room firing an AK-47 assault rifle.
Mercado allegedly killed Tarin, who is the mother of his infant son, and her father and brother during the pre-dawn attack in May. Tarin's mother was wounded in the attack.
Authorities say Mercado and Tarin had been locked in a custody dispute over their son.
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