(CNN) -- A California county sheriff said Friday a couple executed what he called a "cold, calculating plan" to lure a mother to their home, strangle her and then run off with her baby.
Merced County sheriff's deputies Wednesday arrested Teresa Ceja Robles, 33, and her husband, Jose Augustine Velarde, 37, and charged them with murder in the killing of Ana Lila Diaz DeCeja inside their Planada house. The victim's infant boy reunited with his surviving family members, including his father and grandmother, the following night -- nine days after being abandoned in frigid conditions on a stranger's front stoop.
Sheriff Mark Pazin told HLN's "Nancy Grace" the pair killed the 26-year-old woman because they wanted to have her 2-month-old boy, a desire that began after Diaz let Robles hold the baby in a medical clinic shortly after his birth.
Neighbors last saw Diaz and her young son together December 2, getting into a blue 2002 Chevrolet Avalanche in Planada, the sheriff's office said.
Pazin said Robles and Velarde got Diaz to come over to their house, where the mother "was "immediately attacked by the husband, savagely strangled ... and lit on fire to hide the evidence."
A charred body, identified using dental records as that of Diaz, was found later that day in an almond orchard in Snelling, less than a 30-mile drive to the north.
Beyond befriending Diaz, the sheriff said that store surveillance video shows Robles shopping for baby items on the same day -- December 2 -- that she and her husband allegedly kidnapped the boy and killed his mother.
Robles had three children of her own before the incident, and she and her husband even introduced the baby to them. But eventually, the couple decided to change course.
The couple told detectives that they planned to tell Robles' children that the infant had gone to a nearby Fresno hospital and, in a few days, "were going to then tell the children that the little infant had died," according to Pazin.
"They were compounding what they had already done," the sheriff said. "And here, they had wanted to raise this little infant."
Pazin credited "intense media scrutiny" with turning up the heat on Robles and Velarde, who eventually dropped the baby off on a doorstep four days after allegedly kidnapping him.
On Thursday, members of Diaz's family -- including her husband and mother -- gathered for a bittersweet reunion with the baby boy, whom they had not seen in nearly two weeks.
"This is really a joyful moment, and we are really happy to have (the baby) here," the victim's brother Rodolfo Diaz told CNN affiliate KGPE. "But still, we want justice for my sister."