Authorities in Idaho are searching for two children whose disappearance may be tied to a suspicious death investigation and who may be in danger, police said.

Joshua Vallow, 7, and Tylee Ryan, 17, were last seen in September, according to the Rexburg Police Department.

Lori Vallow and Chad Daybell. Freemont County Sheriff's office

Investigators have contacted several of their family members, none of whom said they have been in touch with the children since September, Rexburg police said. The children have not been reported missing by their parents to any law enforcement agencies.

Police have released photos of the children. Joshua has brown hair and eyes, is 4 feet tall, weighs 50 pounds and sometimes goes by the nickname JJ. Ryan has blonde hair and blue eyes, is 5 feet tall and weighs 160 pounds.

Rexburg police are asking for the public's help in locating their mother, Lori Vallow, and their stepfather, Chad Daybell, who are wanted for questioning in connection with their disappearance.

Vallow is Daybell's second wife and may be using his last name, police said.

Neither Vallow nor Daybell is cooperating with the investigation, according to police.

The search for the children began Nov. 26, when police attempted to conduct a welfare check on Joshua, who is adopted and has special needs. Extended family outside of Idaho told police they had not been able to speak with Joshua and were concerned. Rexburg police said they spoke with Vallow and Daybell, who said Joshua was staying with a family friend in Arizona. Investigators later learned that was a lie, police said.

On Nov. 27, Rexburg police executed search warrants at locations in Rexburg associated with Vallow in an attempt to locate Joshua. As the warrants were being executed, investigators determined that Vallow and Daybell had abruptly vacated their home and left Rexburg.

Authorities noted that Vallow and Daybell were married a couple of weeks after Daybell's first wife, Tammy Daybell, was found dead at her home in Fremont County.

Family members found Tammy Daybell, 49, dead Oct. 19. Her death was initially believed to be natural and she was entombed but her remains have since been exhumed in Utah where she was buried. An autopsy will be conducted to determine her cause of death, which authorities now consider suspicious.