Dec. 30, 2010  -- Relatives of the fifth grade boy who was found dead and burned beyond recognition say that they have no idea who the woman is that police have charged with the child's murder.

Mona Nelson is "an acquaintance" of the family of Texas fifth grader Jonathan Foster, whose body was found Monday in a Houston ditch, police said.

Nelson, 44, has been charged with capital murder and is being held without bond. According to ABC News' Houston affiliate KTRK, Nelson did not appear at a court hearing this morning because of an "unspecified medical issue."

Late Wednesday, the severely burned body was finally identified as that of Jonathan Foster, the 12-year-old boy who vanished on Christmas Eve. The cause of death has not yet been determined.

"Nelson has given what investigators say is a self-serving statement that places her with Jonathan, but she has not admitted to killing him," said Houston Police Department spokesman Kese Smith.

Smith said that Nelson is the only suspect in the case.

Police also said their search for Jonathan was delayed by several days because the boy's mother, Angela Davis, and stepfather, David Davis, originally gave cops conflicting information.

Nelson is believed to be the woman who the mother claims answered the phone at her home on Christmas Eve, a chilling call that had authorities concerned for Foster's safety.

Mary Gifford, the boy's grandmother, said that when her daughter called her son back on Friday, a woman she did not know picked up the phone.

"My daughter asked to speak with her son, and then the woman asked [Jonathan] if Angela was his mother," said Gifford. "He said, 'Yes ma'am, Angela is my mother,' and then the phone went dead."

Gifford, who had been holding out hope that her grandson would be found alive, told ABCNews.com today that she's devastated.

"I'm trying to hang in there, but what would be going through your mind if this happened to your child?" she said. "I keep thinking 'Why did she do it? Did she torture him? Why didn't she just let him come home?"

Gifford said she didn't know Nelson and that she wasn't sure if her daughter knew her either.

Was Amber Alert Too Late for Jonathan?

Glenn Scrimsher, Jonathan's uncle who had cared for the boy for four years until November 2009 when he went to live with his grandmother, said today that he had always worried about his nephew living with his mother.

"I was worried," said Scrimsher. "Because of my sister's state of mind and her lifestyle was not that of a caring mother who looks after her children."

Scrimsher said he didn't know Jonathan was living with his mother until after the fact. Foster moved back in with Davis just last month.

"The last time I saw him was the day he was leaving for his grandmother's," said Scrimsher. "He was happy to see his grandmother. He called her his granny."

"He loved her very much and he missed her," Scrimsher said.

Foster's grandmother has said that she believed the Houston police waited too long to issue an Amber Alert for the missing boy. The alert was issued on Monday afternoon, nearly three days since he had last been seen.

"Regardless of how this turns out, my goal is to get the Amber Alert law changed," she said. "There should not be criteria for an Amber Alert that depends on whether cops think the kid has run away."

"The cops kept thinking [Jonathan] ran away," she said. "But he had waited too many years to go back with his mama, that's what he wished for on his birthdays, to be back with his mama... That doesn't sound like a kid who wants to run."

Smith defended the timing of the Amber Alert, saying that conflicting accounts of what happened the day Jonathan went missing made the case even harder for investigators.

While he wouldn't specify what the conflicting stories were about in great detail, Smith said that the mother and the boy's stepfather David Davis had originally said that Jonathan was home with a babysitter when he disappeared, but later said he had been at home alone.

Woman Arrested in Death of Burned Boy

Only when they could pin down an accurate description of the boy, as well as identify the person who might have been with him at the time of his disappearance -- a woman with a raspy voice -- the officers issued an Amber Alert.

"If we issued an Amber Alert on every case, we'd be inundating the public with alerts for children who really did just run away," said Smith.