The couple at the center of a nationwide hunt for two missing Idaho children may be members of a cult preparing for the end of the world, according to reports.

The FBI’s assistance was sought by Idaho police who announced late Saturday that they want to question Chad Daybell, 51, and Lori Vallow, 46, about her children, Joshua “JJ” Vallow, an adopted 7-year-old autistic boy, and Tylee Ryan, a 17-year-old girl, who have not been seen since September.

Police said Vallow and Daybell have refused to help them find the children and are now on the run.

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Brandon Boudreaux, a relative, told Fox 13 Salt Lake City that Vallow joined a “cult” with Daybell, a self-proclaimed Latter-day Saint author known for his religious books detailing the end of the world.

The religious group is an offshoot of The Church of Jesus Christ of Latter-day Saints, with more radical beliefs, the station reported.

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Boudreaux told the station his wife, Vallow’s niece, also ran off with Vallow to join Daybell’s religious group.

“Their focus is just so much on the end of the world, the idea that it’s coming,” he said, according to the station.

Police weren't saying if they believe that Vallow and Daybell's religious beliefs are connected to the case, Fox 13 reported.

Police in Rexburg, Idaho, began searching for Joshua and Tylee after Daybell’s 49-year-old wife Tammy died in October at home. Initially, police ruled she died of natural causes but now consider the death suspicious and exhumed her body in Utah last month as part of the investigation.

Tammy's father Ron Douglas told the station that he learned she died when Chad Daybell called him “broken-hearted and sobbing” to say she went to bed after a coughing fit and never woke up.

“We speculate, like everybody else,” Douglas said. “It’s hard to know that we lost our daughter and that our son-in-law of 30 years has stepped into this mess. We don’t know. We’d like to hope for the best. Every time you peel a layer off the onion, it makes you scratch your head.”

Police believe Daybell married Vallow soon after his wife’s death.

Vallow's own husband was fatally shot in July during a confrontation with Vallow’s brother who claimed self-defense and was never arrested.

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The brother Alex Cox died less than two weeks ago and police in Gilbert, Ariz., are investigating his death, Fox 10 Phoenix reported.