Their mother, Kelli Murphy, 41, called 911 on May 23 to say that she had attempted suicide. When the dispatcher asked about the kids in the home, Murphy replied that they were "already in heaven," according to Castle Rock Police Chief Tony Lane.

9-year-old Liam and 6-year-old Madigan were found dead in the family's home.

Toxicology tests on the children came back negative, the coroner said.

An autopsy by forensic pathologist Dr. Michael Burson found the children were smothered. He ruled the manner of death for both children was homicide.

Murphy was charged with four counts related to first-degree murder -- two counts of murder after deliberation and two counts of murder where the victim was under 12 years old.

She was also charged with two counts of child abuse, knowingly and recklessly causing death.

Liam & Madigan

Liam was known for his love of race cars and often helped his friends with their math homework.

"He was like the math genius in school, so all the kids would go to him," said neighbor CeCe Roulge.

Madigan was known for her kindness and sense of humor.

"She was hilarious. She made people laugh. She had jokes all the time," Roulge said.