Investigators are considering searching inside Yellowstone National Park for, sources close to the investigation told CBS News. Ryan, 17, and her 7-year-old brother Joshua "JJ" Vallow haven't been seen since September.
Their mother, Lori Vallow,and faces child abandonment and desertion charges.
According to court documents, police in Rexburg, Idaho, said phone records show Ryan visited Yellowstone with her mother, uncle and brother JJ on September 8, CBS News correspondent Jonathan Vigliotti reports. Police also "obtained a photo from Lori Vallow's iCloud account" of Ryan in the park. They said there's been no sign of life since.
Detectives are waiting for the snow to melt in Yellowstone to search, the sources said. Park officials declined to comment citing an "active investigation."
Police saidshowing what is believed to be JJ playing with a neighbor in September just days before he disappeared.
Lori Vallow and her husband Chad Daybell had left Idaho after police conducted a welfare check at their home in November and were found in Hawaii in January. Police said they have lied about the children's whereabouts.
Former Idaho Attorney General David Leroy said the state has never seen a case like this.
"This is a very atypical defendant and a very unusual case," he said. "The implication of a parent who will not cooperate in disclosing their location ... is that something very untoward has happened to these children."
Both of Lori Vallow and Daybell's former spouses, as well as Vallow's brother, died under what authorities called suspicious circumstances in the last year. Vallow and Daybell have denied any wrongdoing.
The ex-husband of Vallow's niece claims in divorce filings that his ex-wife knows where the children are and was involved in a cult, where numerous members, both adults and children, were killed.
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