( CNN ) -- A California county sheriff said on Friday that a couple executed what he called a `` cold , calculating plan '' to lure a mother to their home , strangle her , 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 murdering 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 '' that the pair killed the 26-year-old woman because they wanted to have her two-month-old boy , a desire that began after Diaz let Robles hold the baby in a medical clinic shortly after his birth . `` That 's when she fell in love with the infant and had to have that baby , '' Pazin said of Robles . `` The only word I can use to categorize ( their actions ) is despicable . It 's just bizarre . '' 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 that 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 . Pazin said Friday that the couple had `` an absolute plan ( for ) every move they made . '' 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 KPGE . `` But still , we want justice for my sister . ''
NEW : A California sheriff says a couple schemed to kill a mother and steal her baby . Merced County 's sheriff said the victim was strangled to death , dumped , burned . Police say they 're working to determine whether the couple tried to sell a baby . Score: 6