The aunt of one of the two missing Idaho kids said their mom, Lori Vallow, had recently become obsessed with the end of the world. Her husband, who she married weeks before 7-year-old JJ Vallow and 17-year-old Tylee Ryan were last seen, has written multiple books about doomsday scenarios.

Chad Daybell self-published more than 25 books from his Idaho home, many about his brush with death and the end of the world.

"I recently released my autobiography where I tell more about my two near-death experiences," Daybell said at a conference in 2017.

His preachings are what first attracted Vallow.  

"And she says, sometimes, I think it would be better just to put my kids in a car and go off the side of a cliff," Ryan's aunt, Annie Cushing, told NBC affiliate KSL in a Skype interview.

Vallow moved to Idaho to be with Daybell, after her previous husband, Charles Vallow, died of suspicious causes last summer in Arizona. Daybell's wife, Tammy, also died under mysterious circumstances last fall.

Len Humphries, the sheriff in Freemont County, Idaho, is investigating Tammy's death and said it is possible that Lori Vallow could have played a role, along with Chad.

"There's a chance that that has occurred and one of the things that we are looking at very closely," he told CBS News correspondent Jonathan Vigliotti.

Tammy Daybell's body has been exhumed, but results for the autopsy haven't been released.

Last month, authorities confiscated 43 items from Chad Daybell's Idaho home including cellphones, journals, documents and medications.

Asked if there have been any signs of life of the children, Humphries said no. 

"You would expect parents, and specifically Lori as a mother, to be a little more responsive about what's going on with their children," he added.

There are no signs of life, but also no signs of death, and without that, investigators said it's nearly impossible to bring the couple back to Idaho. They are currently in Hawaii and have denied any wrongdoing.