Investigators are still searching for 17-year-old Tylee Ryan and 7-year-old Joshua "JJ" Vallow. Authorities determined that Tylee was last seen with her mother, Lori Vallow, and her uncle, Alex Cox, at the sprawling park on Sept. 8.
Joshua Vallow was last seen Sept. 23. Investigators are looking for anything that shows Vallow, Cox, the children or a 2017 silver 2017 Ford F-150 pickup with Arizona license plate CPQUINT.
The group was traveling in the vehicle to Yellowstone, the FBI said.
“Law enforcement is seeking photos and video which may have captured images of these persons and the vehicle, or images of crowds and other park visitors where these individuals may be present,” an FBI statement said.
Lori Vallow, 46, was arrested in Hawaii last month and charged with felony child abandonment, in addition to resisting or obstructing police officers, solicitation to commit a crime and contempt of court. The arrest stemmed from her noncompliance with a court order to produce the children for Idaho welfare workers or Rexburg, Idaho, police.
She waived her right to extradition and will be transferred to Idaho to face the charges.
Police in Rexburg said they believe the children's lives are in danger and that Vallow and her husband, Chad Daybell, have lied about the children's whereabouts.
Vallow's former husband, Charles Vallow, was fatally shot by Cox in Phoenix last year. Cox claimed self-defense and was never arrested. He died of unknown causes in December.
Before his death, Charles Vallow accused his wife in divorce papers of joining a doomsday cult and declaring herself a god sent to prepare for the end of the world. He also said in court documents he feared his wife would kill him.
Daybell's former wife, Tammy Daybell, died unexpectedly in their home in October. He married Vallow two weeks after the death, arousing suspicion from law enforcement. Tammy Daybell's remains have been exhumed but an autopsy report has not been released.
Vallow and Daybell, her fifth husband, fled Idaho for Hawaii in November after authorities began asking questions about the missing children. Her family believes she may have joined a doomsday cult. Daybell is the author of religious-themed fiction books about the biblical end-times.
She reportedly believes she is “a god assigned to carry out the work of the 144,000 at Christ’s second coming in July 2020,” according to divorce documents that Charles Vallow filed before his death.
Photos and videos can be uploaded at FBI.gov/Rexburg.
Fox News' Danielle Wallace and The Associated Press contributed to this report.