Police in southeast Georgia are trying to determine how two children could go missing for up to two years without anyone reporting it to the police – following the grisly discovery of the bodies of a 14-year-old brother and sister in their father’s backyard.

The Effingham County Sheriff’s Office was responding to a welfare check for Mary Crocker on Dec. 19 when investigators discovered her body and the body of her brother, Elwyn Crocker at the home in Guyton, Georgia.

Police said although Mary had not been seen in weeks and her brother Elwyn Jr. had not been seen since November 2016. Neither child had been reported missing to authorities.

Autopsy reports confirmed that Elywn Crocker, Jr., who would be 16-year-old by now, had been buried for at least two years, while Mary had been buried for months.

Police arrested the children’s father, as well as their step-mother and step-grandmother and her boyfriend. All four have been charged with concealing a death and cruelty to children.

The children’s father, Elwyn Crocker Sr., 49, dressed up as Santa at a local Walmart until recently, the sheriff’s office said, according to the Atlanta Journal-Constitution.

During the course of the investigation, detectives found a special needs child the home. The child was taken to the hospital for observation.

Effingham County Coroner David Exley said the cause of their deaths has not yet been determined. Exley said authorities are still determining who to release the bodies to and are still trying to contact the children’s mother.

On Thursday, police charged Elwyn Crocker Sr.; the children’s stepmother, 33-year-old Candice Crocker; and their step-grandmother 50-year old Kim Wright with concealing the death of another and cruelty to children in the first degree.

The following day police arrested and charged Wright’s boyfriend, 55-year-old Roy Anthony Prater with concealing the death of another, cruelty to children in the first-degree and possession of a scheduled or controlled substance.

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Write to Gina Martinez at gina.martinez@time.com.