Tami Joy Huntsman, 39, and Gonzalo Curiel, 17, have already been accused of horrific crimes.
The couple is already charged with felony child abuse and torture in the case of a 9-year-old girl who was so badly injured she had to be hospitalized, authorities said.
Now they've suspects in a more shocking crime: The killing of two preschoolers found dead in a rented storage unit, said police in Redding, California.
On Sunday, Redding officers acted on a tip from another police department, went to the AAA Enterprise Store-All and found a locked unit, police said. They broke the lock and inside discovered the two dead children.
Redding police launched a homicide investigation, saying Huntsman and Curiel are considered suspects.
"This is a very fluid, complicated and ongoing case stemming from what appears to be a series of horrific acts," said David Hollister, the district attorney in Plumas County, where the couple has lived.
Redding police have not identified the dead children yet.
The department noted Huntsman and Curiel had physical custody of two children, brother and sister Delylah Tara, 3, and Shaun Tara, 6, who have been reported missing.
Huntsman and Curiel moved around the past few weeks, traveling to Redding; Plumas County, in northeast California; and Monterey County on the coast, the Redding police said in a news release.
On Friday, the Plumas County Sheriff's Department received a call about a possible case of child abuse or neglect in Quincy, a town of about 1,700 people, authorities said. They went to the place where Huntsman and Curiel were staying.
Plumas County Sheriff Greg Hagwood said officers found a 9-year-old girl on the floorboard of a locked SUV, according to The Plumas News. She weighed about 40 pounds, had open sores, lice, broken bones and missing teeth, the sheriff said.
"This has shaken my staff to the core," Hagwood told the Plumas News. "That little girl had been subjected to the most unspeakable measure of torture for an extended period of time. This is child abuse, the likes of which we haven't experienced here (in Plumas County)."
The girl was taken into protective custody and is receiving medical care in Sacramento hospital, the Redding police said.
Two other children, 12-year-old twins living with Huntsman and Curiel, were placed in foster care, the Plumas News said.
Huntsman is the 9-year-old girl's aunt and had custody of the girl; Huntsman is the biological mother of the 12-year-old twins, the Plumas News said.
Huntsman and Curiel were arrested and charged with torture and felony child abuse.
On Sunday, the Plumas County Sheriff's Office got a call from someone in Salinas, in Monterey County, asking about the two preschoolers, according to the Sacramento Bee.
"We knew nothing about a 6-year-old and a 3-year-old," Hagwood said. The two children had not been seen when the child abuse investigation started.
Deputies interrogated Huntsman and Curiel again and learned about the storage unit, the Bee reported. They called police in Redding.
Huntsman is not the mother of Delylah and Shaun, but her relationship to those children is undetermined, a social services official told CNN.