REXBURG, Idaho —One of the country’s leading experts studying cult behavior is now weighing in on the search for two missing children in Rexburg, Idaho.

The FBI and police agencies across the country are continuing to search for 17-year-old Tylee Ryan and her autistic brother, seven-year-old Joshua Vallow. The two children have been missing since September.

Officers with the Rexburg Police Department said the children’s newlywed parents Chad Daybell and Lori Vallow did not report the children missing, fled from Idaho, and refuse to cooperate with the investigation.

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TIMELINE: Attorneys I've spoken to say, based on this timeline and the facts investigators have been able to gather so far, police have enough evidence to arrest Chad Daybell and Lori Vallow today for obstruction... but they might be strategically choosing not to do so.@fox13 pic.twitter.com/wzcnP15bYE — Adam Herbets (@AdamHerbets) January 2, 2020

Family members of Lori Vallow tell FOX 13 she was a great mother until something changed, perhaps when she met Chad Daybell.

Lori Vallow’s husband, Charles Vallow, was shot and killed by her brother Alex Cox in July. Cox told officers the shooting was in self-defense, and the case is under investigation by the Chandler Police Department.

Alex Cox later died in December. The circumstances of his death are not being released. The case is under investigation by the Gilbert Police Department.

RELATED: Family members say mother of missing children in Rexburg joined a dangerous cult with new husband

Medical examiners initially believed Chad Daybell’s wife, Tammy Daybell, died of natural causes in October.

Within two weeks, Chad Daybell married Lori Vallow.

Tammy Daybell’s remains, which were buried in Springville, Utah, have since been exhumed for further testing due to potentially suspicious circumstances. The case is under investigation by the Fremont County Sheriff’s Office.

Rick Alan Ross, the executive director of the Cult Education Institute, said he agrees with family members who are deeply concerned with the newlywed couple’s religious beliefs.

“(Lori Vallow’s) husband dies. Her brother dies, and the children disappear,” Ross said. “Something is going on. It’s very dark. It’s very sinister, and in my opinion, very cult-like.”

Ross said he believes Chad Daybell’s books – focused on the end of the world – are a major red flag.

“It seems to me that Chad Daybell is the leader, and he’s the defining element of this group. He is the one who has created this whole belief system around his revelation,” Ross said. “I mean, he wrote 25 books… He seems to use his LDS background as a recruitment tool… It seems to me, according to the family, that Lori Daybell became increasingly dependent on Chad Daybell and was his ardent follower and supporter.”

Julie Rowe, a friend of Chad Daybell, told FOX 13 in December that she has not maintained contact with Chad Daybell but believes he would never hurt his wife or stepchildren. She confirmed Chad Daybell received visions that his wife would die.

“I did ask him, ‘Chad? Do you still see Tammy dying?’” Rowe said. “He said, ‘Yes I do.’”

Ross said he sees similarities between Chad Daybell and Jeffrey Lundgren, a prominent cult leader who recruited members from the FLDS Church and was executed in 2006.

“(Lundgren) also saw himself as a prophet. He grew a group of people away from the church and isolated them, manipulated them, and it ended with the deaths of five people. A mother, a father, and their three children,” Ross said. “It (also) reminds me of a group in Denver years ago that was called Concerned Christians. They also were defined by doomsday beliefs of their leader, and they just disappeared. To this day, the leader and many of his followers have never reappeared.”

Although Chad Daybell has marketed himself as a member of the Church of Jesus Christ of Latter-day Saints, a spokesperson for the Church has not confirmed or denied whether Chad Daybell has been excommunicated or is still a member.

“All of the major religions, be it Christianity, Hinduism, Buddhism, Islam, all have splinter groups,” Ross said. “Children have no choice when it comes to cults. They are brought in by their parents… I hope this has a happy ending.”

If you have any information about this case, please call the FBI or the Rexburg Police Department.