'What Happened to JJ and Tylee': How to Watch the Lori Vallow Dateline Special Tonight

Amid new developments in the investigation of two missing Idaho children, Dateline is set to air a new Lori Vallow case update. What Happened to JJ and Tylee is scheduled to debut on Monday at 10 p.m. ET on NBC. Episodes of the true crime series are also typically made available for viewing online shortly after they air.

The special is set to air just two weeks after the bodies of Tylee, 17, and JJ, 7, were discovered on the property of their stepfather, Chad Daybell, in Rexburg, Idaho months after they had last been seen alive in September. The special will feature an interview with Vallow's former best friend, Melanie Gibb, marking her first network television interview since the investigation began. Speaking with NBC News' Keith Morrison, Gibbs said that Vallow and Daybell "had deception in them" and said that she "saw him as the hand and her as the puppet on that hand." She said that "they were both, like, gasoline and fire – equally destructive to each other."

At this time, details surrounding the deaths of the two children are unclear, though their deaths were proceeded by a string of mysterious deaths in the family. The children had last been seen alive in early September – Tylee on Sept. 8 on a family trip to Yellowstone National Park and JJ at school in Idaho just a few days later – and were reported missing in late November. Shortly after police began their investigation, Vallow and Daybell had fled to Hawaii, where they married just weeks after the death of Daybel's ex-wife, which is now considered suspicious and for which he is under investigation for.

Vallow was arrested on Feb. 20 after she defied a court order to produce her children to authorities by Jan. 30. Her arrest, however, provided few clues regarding Tylee and JJ's disappearance, and much remained a mystery until authorities executed a search warrant on Daybell's property on June 9. According to the Huffington Post, search warrants suggested that their uncle, Alex Cox, had visited the property at least four times in September, with a Sept. 9 visit being highlighted as "significant," a probable cause affidavit made public Friday showed.

During their search of the property, authorities discovered two sets of human remains in two separate locations that were later confirmed to be that of Tylee and JJ. The 7-year-old had reportedly been bound with duct tape before being wrapped in a plastic bag, while Tylee's remains had been dismembered and burned, according to authorities.

Daybell was later arrested on two felony counts of destruction, alteration or concealment of evidence. Daybell's attorney has said that his client will plead not guilty. It is unclear if either Daybell or Vallow will face additional charges. They are both being held on a $1 million bond.