Authorities are searching for Idaho siblings who were last seen in September and whose mother and stepfather have refused to cooperate in locating them, according to police.
"We strongly believe that Joshua and Tylee's lives are in danger," Rexburg Police Chief Shane Turman said in a statement Monday.
Joshua "JJ" Vallow, 7, and Tylee Ryan, 17, were last seen on Sept. 23., the Rexburg Police Department said in a statement. Police conducted a welfare check on the family's home on Nov. 26 after extended family told authorities they hadn't heard from JJ in weeks.
When police arrived at the home, they said the children's mother, Lori Vallow Daybell, and her new husband, Chad Daybell, told them JJ was staying with family friends in Arizona.
Investigators learned later that day that JJ wasn't in Arizona, police said, and returned with a search warrant for the couple's house the next day. But Chad and Lori "had abruptly vacated their residence and left Rexburg," according to police.
Since then, "Lori Vallow/Daybell, the adopted mother of Joshua and the biological mother of Tylee, has completely refused to assist this investigation," Turman said in his statement.
"We know that the children are not with Lori and Chad Daybell and we also have information indicating that Lori knows either the location of the children or what happened to them," Turman added. "Despite having this knowledge, she has refused to work with law enforcement to help us locate her own children. Lori Vallow has chosen instead to leave the state with her new husband."
The FBI is assisting in the search for the missing children. On Friday, officers from the Rexburg Police Department, the FBI's Salt Lake City office and the Fremont County Sheriff's Office executed a search warrant at the home owned by Chad Daybell and his deceased wife, Tammy, EastIdahoNews.com reported.
Joshua last attended Kennedy Elementary School in Rexburg on Sept. 23, authorities said. He has brown hair and brown eyes, is 4 feet tall and weighs 50 pounds. He has autism and "may be in need of medical attention," according to authorities.
Tylee Ryan has blonde hair and blue eyes, is 5 feet tall and weighs 160 pounds. Neither Joshua nor Tylee had been reported missing to any law enforcement agency before police launched their search following the Nov. 26 welfare check.
Rexburg police are also trying to locate Chad and Lori, calling them "persons of interest" in the case. To date, no charges have been filed against the couple.
An attorney for the couple, Sean Bartholick, issued a statement on behalf of the couple on Dec. 23.
“Chad Daybell was a loving husband and has the support of his children in this matter. Lori Daybell is a devoted mother and resents assertions to the contrary. We look forward to addressing the allegations once they have moved beyond speculation and rumor," Bartholick wrote in the statement.
Bartholick did not respond to InsideEdition.com's request for comment.
Both Chad and Lori's previous spouses have died in the past several months. On Oct. 19, Chad's ex-wife, Tammy Daybell, 49, was found dead in her home, police said. Her death was initially believed to be from natural causes. Chad Daybell married his new wife, Lori, just a few weeks later.
A later investigation "determined that [Tammy] Daybell's death may be suspicious," according to police. Her body was exhumed and an autopsy was performed, the results of which are still pending.
Lori Daybell's ex-husband, Charles Vallow, was shot and killed by Lori's brother, Alex Cox, on July 11, The Arizona Republic reported. Cox died on Dec. 12, and his cause of death has not been made public, according to the paper. An investigation into his death is ongoing.
In his statement Monday, Rexburg Police Chief Turman said, "We are aware in the weeks after Tammy Daybell's death, Lori Vallow/Daybell and Chad Daybell told witnesses that Lori's daughter had died a year before the death of her father, which is untrue. Around that same time, Chad told another witness that Lori had no minor children."
Chad is a member of the Church of Jesus Christ of Latter-day Saints. He has published books about his faith and near-death experiences, as well as fictional stories set in an apocalyptic United States. In the description for another book of "true graveyard stories," Chad wrote he once served as a cemetery sexton.
Both Chad and Lori have appeared on podcasts made by Preparing A People, an organization whose mission is to "help prepare the people of this earth for the second coming of Jesus Christ."
"We were shocked by the reports about Mr. Daybell and the missing children. Like everyone, we are deeply disturbed over media reports involving Mr. Daybell and his new wife Lori Vallow and her 2 missing children," the organization's founders, Michael and Nancy James, wrote on their website.
"We also do not share any of Chad Daybell's or Lori Vallow's beliefs if they are contrary to Christian principles of honesty, integrity and truth, or if they do not align with the doctrines of The Church of Jesus Christ of Latter-day Saints," the statement continued.
The search for the missing children is ongoing. The Rexburg Police Department asks anyone with information regarding the children's whereabouts or welfare to contact the department at (208) 359-3000 or the National Center for Missing and Exploited Children (NCMEC) at 1-800-THE-LOST.