But others believe that criminal charges against sect members could be jeopardized if the girl is never identified.
Lisa Wayne, a criminal defense lawyer, said sect members potentially face a variety of criminal charges, including statutory rape, abuse or negligence.
But the sect could challenge the search warrant that police used to enter the compound if the girl is never identified and some of the evidence that investigators found could be suppressed, she said.
"In a criminal case, you have the right to know who may give the government probable cause to come on the premises," she said.
In her calls to the family shelter, Sarah said she believed she was pregnant with her second child. She claimed her husband was a 49-year-old man named Dale. She said that he raped and beat her, once breaking her ribs, and that she was being held at the ranch against her will.
This weekend, Texas Rangers interviewed Dale Evans Barlow, 50, who is named in the search and arrest warrant used to search the compound, but did not arrest him.
Barlow was briefly jailed in Arizona last year, sentenced to three years' probation and forced to register as a sex offender after he was convicted of conspiracy to commit sexual conduct with a minor, according to Mohave County, Ariz., Probation Department Chief Friend Walker.
But, Barlow's lawyer told ABC News that he has not been to Texas since 1977 and Walker said Barlow had been checking in regularly with his Arizona probation officer.
Kathleen Mackert, who said she fled the FLDS community in Colorado City after years of sexual abuse, said she believes Sarah is real but may have been taken out of the compound before police arrived.
"They would want to avoid them getting ahold of her at all costs," she said.
Mackert said sect members are taught that they should lie to the government and other outsiders. "We were expected to lie," she said. "They were the enemy."
Sarah's absence probably will not affect today's custody hearing, during which the state will ask to keep the children in state custody and place them in temporary foster homes, legal experts say. More important to the future of the children in state custody will be what the police found once they were inside the compound.
Guy Choate, who is helping to coordinate lawyers for the children, said it's not unusual for child custody cases to be based on an anonymous call. If caseworkers find evidence of abuse, they are obligated to take action, he said.
Authorities say in court records that there is a widespread practice within the sect of marrying off underage girls to older men. Investigators have found several girls who appear young who are either pregnant or have children, the records say.
More than 350 lawyers are expected to descend on the Tom Green County Courthouse this morning, when a judge will begin to sort out which of the children should be sent to foster homes and which, if any, will be sent back to their parents.
The nearly unprecedented case appears to be the largest child protection case in U.S. history and one that threatens to overwhelm the judges, lawyers and government officials working on it. Each child is entitled to a lawyer, and many of the parents are expected to have lawyers speaking on their behalf as well.
"We've never really seen anything like this," said Lynne Gold-Bilkin, a family law expert.