The mother of two missing siblings in Idaho is refusing to cooperate with the investigation and left the state with her new husband, authorities said.

Police have been searching for Joshua Vallow, 7, and Tylee Ryan, 17, since November when officers with the Rexburg Police Department attempted to conduct a welfare check on Joshua, who is adopted and has special needs.

Extended family outside of Idaho contacted police because they had not heard from Joshua and had become concerned. Authorities said the children were last seen in September and they may be in danger.

Last week, investigators asked for the public's help in locating the children's mother, Lori Vallow, and their stepfather, Chad Daybell. In a statement on Monday, police said Vallow "has completely refused to assist this investigation" and is no longer in the state of Idaho.

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

"It is astonishing that rather than work with law enforcement to help us locate her own children, Lori Vallow has chosen instead to leave the state with her new husband," Rexburg police said in a press release posted on their Facebook page.

According to authorities, Vallow and Daybell have repeatedly lied to investigators and witnesses. Police previously said that Vallow told officers in November that Joshua was in Arizona with a family friend, but detectives later learned that was a lie.

Both Vallow and Daybell also falsely told a witness that Tylee had died a year before the girl's father was killed in July. Charles Vallow, Lori's Vallow's first husband, was shot to death by her brother following a confrontation. Chandler police in Arizona said Lori Vallow and her brother were questioned by police but were not charged.

Rexburg police said they have learned that Vallow either knows the location of Joshua and Tylee or knows what happened to them.

"We publicly call on Lori Vallow/Daybell and Chad Daybell to do the right thing and come forward with the information they have about the location and welfare of Joshua and Tylee," police said. "This entire investigation could have been avoided if Lori and Chad had simply been honest with law enforcement."

Police said earlier in December that they believe the children's disappearance could be tied to the death investigation of Daybell's first wife, Tammy Daybell.

According to police, family members found Tammy Daybell dead in her home in October. The 49-year-old's death was initially ruled as natural but has since been classified as suspicious, and her remains were exhumed so an autopsy could be conducted.

Weeks after Tammy Daybell's death, Chad Daybell and Vallow married, police have said.