Authorities hunting for 'cult' mom Lori Vallow's missing children are preparing to search Yellowstone National Park as soon as the snow melts, sources say.

Lori's daughter Tylee Ryan, 17, was last seen at an entrance to the park on the border between Idaho and Wyoming in early September. Tylee's seven-year-old brother Joshua 'JJ' Vallow disappeared three weeks later from their home in Rexburg, Idaho.

The children were never reported missing by their mother, who was arrested in Hawaii last week on charges of child abandonment and desertion.

Sources familiar with the multi-state hunt for JJ and Tylee told CBS News that detectives are waiting for snow to melt in Yellowstone before they begin searching the 3,500-square-mile park for signs of the elder child.

Snow usually sticks on the park grounds until May, sometimes June. Park officials declined to comment due to the 'active investigation'.

Court documents released two days before Lori was arrested on Thursday revealed that phone records showed Tylee visited Yellowstone with her mother, her brother and their uncle, Alex Cox, on September 8.

Police also recovered a photo of Tylee in the park from Lori's iCloud account. They said there has been no sign of the 17-year-old since.

Authorities hunting for 'cult' mom Lori Vallow's missing children are preparing to search Yellowstone National Park (above in a file photo) as soon as the snow melts, sources say

Lori's 17-year-old daughter Tylee Ryan (pictured) was last seen at an entrance to the park on the border between Idaho and Wyoming on September 8

Tylee's seven-year-old brother Joshua 'JJ' Vallow (left) disappeared three weeks later from their home in Rexburg, Idaho. The children were never reported missing by their mother Lori (right), who was arrested in Hawaii last week on charges of child abandonment and desertion

JJ was last seen at his elementary school in Rexburg on September 23, a day before his mother withdrew him and told administrators she would be homeschooling him.

Doorbell camera footage recorded at a neighbor's house appeared to show JJ playing in his front yard on September 17.

Authorities began searching for the children in late November after performing a welfare check ordered by concerned relatives who said they hadn't spoken to JJ, who is autistic, in months.

When officers first went to Lori's home in Idaho on November 26, she told them that JJ was visiting relatives in Arizona - which investigators say was a lie.

Officers returned the following day and found that Lori and the man she married weeks earlier, Chad Daybell, had fled from the home.

Authorities say the couple have repeatedly lied about where JJ and Tylee are and refused to cooperate with the investigation.

It has since emerged they had told people Tylee was dead and that Lori has no minor children.

The last known footage of JJ, recorded on a neighbor's security camera, showed the seven-year-old playing outside his home in Rexburg on September 17

Police are seen frisking Lori before she was taken into custody outside her Kauai apartment

As officers prepared to take Lori away in a patrol car, her Doomsday author husband Chad Daybell (right) watched helplessly

Timeline of JJ and Tylee's disappearance July 11: Lori Vallow's husband, Charles Vallow, is killed by her brother, Alex Cox, in Arizona August: Vallow moves children JJ and Tylee to Rexburg, Idaho September 8: The last time Tylee was seen on a day trip to Yellowstone park September 23: The last time JJ was seen at his school in Idaho October 19: Chad Daybell's wife Tammy dies at their Idaho home October 25: A friend of Tylee receives a text from her phone November 5: Vallow and Daybell marry November 26: Out-of-state relatives ask Idaho police to perform a welfare check on JJ. Vallow and Daybell claim he is in Arizona with relatives. Police also learn Tylee has not been seen since September, either November 27: Police execute a search warrant at Vallow and Daybell's home, discovering the couple have fled Idaho December 11: Tammy Daybell's body is exhumed from the Utah cemetery December 12: Vallow's brother, Alex Cox, believed to have died in Arizona December 21: Police issue a press release about JJ and Tylee, revealing they believe their disappearance is linked to Tammy's death December 24: Vallow and Chad issue a statement through an attorney saying they love their son and daughter and look forward to addressing 'allegations once they have moved beyond speculation and rumor' December 30: Police accuse Vallow and Chad of lying to investigators and say they believe the couple know where the kids are or what happened to them January 26: Vallow and Chad are seen for the first time in months as police serve two search warrants in Kaua'i January 30 Vallow misses court deadline to produce the children to authorities Advertisement

Lori and Chad were named persons of interest in the children's disappearance after investigators said they believe the mother knows where her children are or what happened to them.

The couple issued a statement after they went on the run insisting that they'd done nothing wrong.

Police have said there is reason to believe the children are in danger.

The case captured nationwide attention with the revelations that police are also investigating three mysterious deaths linked to Lori and Chad, as well as family members' claims that the couple are members of a dangerous doomsday cult.

The first death is that of Lori's estranged husband Charles Vallow, who was shot dead by her brother Alex Cox in Arizona on July 10.

Charles and Lori had gotten into an argument when the father came to pick up JJ at the mother's home in Chandler.

Lori's brother intervened and fatally shot Charles.

Police initially determined that he acted in self defense - but the case was reopened amid the multi-state search for JJ and Tylee, who had moved to Idaho, where Chad lived, with their mother in August.

The second mysterious death was Tammy Daybell - Chad's previous wife - who was found dead at the couple's home in Salem, Idaho, on October 19.

An obituary stated that Tammy passed away in her sleep and her cause of death was ruled as natural after Chad reportedly declined an autopsy.

Investigators reopened the case after learning that JJ and Tylee were missing, as their mother had married Chad just two weeks after Tammy died. They believe the two cases could be linked.

Tammy's body was exhumed on December 11 and the autopsy results have not yet been released.

On December 12, Lori's brother, Alex Cox, was found dead in Gilbert, Arizona.

The 51-year-old's death is now under investigation as police wait for an autopsy to determine the cause.

Lori's estranged husband Charles Vallow (pictured together) was shot dead by her brother Alex Cox in Arizona on July 11

Chad's wife Tammy Daybell (pictured together) was found dead under mysterious circumstances in October, just two weeks before he married Lori

Lori's brother Alex Cox (pictured) died under mysterious circumstances on December 12

Police tracked Lori and Chad down in Princeville, Hawaii, late last month and served the mother with a court order requiring her to physically produce the children to authorities in Idaho by January 30.

Lori failed to meet the deadline and was arrested last Thursday in Kauai.

She is currently jailed in lieu of $5million bail as she awaits extradition to Idaho, which officials say could take up to 90 days.

She is due to appear in court on March 2 for the next step in the extradition process.