The victim, Alexis Rasmussen, died of an apparent overdose the night of Sept. 10, 2011, after she was injected with methamphetamine and heroin by Eric Millerberg, Dea Millerberg's husband.
Weber County Attorney Dee Smith said Wednesday that Dea Millerberg's guilty pleas were a "fair resolution" to the case, since Millerberg testified against her husband at his February trial.
"She provided a lot of information," Smith said. "A lot of testimony that allowed us to solve this."
Eric Millerberg, 38, was sentenced to life in prison in March. He was convicted of child-abuse homicide, obstructing justice, desecrating a human body and having unlawful sexual activity with Alexis.
During Eric Millerberg's three-day jury trial, Dea Millerberg took the stand and testified against her husband of nearly 10 years.
She recounted that on Sept. 10, 2011, she picked up Alexis to baby sit their kids, but instead she got high with the teen and her husband.
Through the night, Eric Millerberg injected Alexis with drugs three times, Dea Millerberg testified once in the teen's neck. The three also attempted to have sex together, the woman testified, but they were too high.
Shortly after her husband injected Alexis for the third time, Dea Millerberg testified that the teen began to complain of being cold and shaky. The teen took a bath, and afterward, the Millerbergs helped her into the couple's bed, then the adults went outside to smoke.
When they returned, they found Alexis was not breathing. Dea Millerberg, who was at the time a licensed nurse, said she attempted CPR, but the girl never responded.
In a panic, the Millerbergs weighed their options. Ultimately, they decided to dump the teen in a remote part of Morgan County near the Taggart exit of Interstate 84.
Alexis was missing for 38 days before Eric Millerberg's friend, Eric "Peanut" Smith, broke the case for police. He testified that he helped Eric Millerberg move the girl's body on Sept. 12, 2011 a day after the Millerbergs initially dumped it.
Smith led police back to the girl's badly decomposed body in October 2011.
The girl was found with her legs folded against her in a fetal position, her lower half stuffed into a garbage bag.
Dea Millerberg was granted "use immunity" in her husband's trial, meaning her testimony couldn't be used against her.
Charges were never filed against Smith, who made a deal with police in exchange for leading them to the girl's body.