The inmate would later tell police that Millerberg had admitted giving Alexis Rasmussen a deadly mixture of methemphetamine and heroin.
"There are rules," the inmate said of prison culture. "You don't mess with children. You don't mess with females. [Millerberg] broke every code there is, and he was running around like it was a big game."
Millerberg is accused of injecting the teen with drugs on Sept. 10, 2011, at his North Ogden home. After the girl died of an apparent overdose, Millerberg and his wife, Dea Millerberg, decided to get rid of the body, prosecutors said.
Eric Millerberg, 38, is charged with first-degree felony child-abuse homicide and felony counts of obstructing justice, desecrating a body and having unlawful sexual activity with a minor.
On Wednesday, 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 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 went outside to smoke.
When they returned, they found that Alexis was not breathing. Dea Millerberg 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.
On Thursday, Eric "Peanut" Smith testified that Eric Millerberg called him on Sept. 12, 2011, asking for help. Together, they drove up Weber Canyon to where the girl's body had been left in a wooded area.
"He told me that he needed my help real fast because he didn't feel safe where [Alexis' body] was at," Smith testified.
They attempted to dig a hole, but the ground was too rocky. Instead, Eric Millerberg shoved the girl's body into a garbage bag and pulled her 20 yards farther into the trees, Smith said.
Thirty-eight days later, Smith, handcuffed and in a jail jumpsuit, led police back to the girl's badly decomposed body. The girl was found with her legs folded against her in a fetal position, her lower half still stuffed into the garbage bag.
After Alexis' mother filed a missing-persons report, Eric Millerberg told Alexis' mother and police that the teen left his house late in the evening on Sept. 10, 2011. He said he had no idea where the girl could be.
North Ogden police Detective Bill Aeschlimann testified Thursday that after the girl's body was found, Eric Millerberg, who was in custody for violating his probation in a 2010 credit card fraud case, still claimed to be ignorant of the girl's fate.
Aeschlimann said he told the man they had found the girl.
"Is she OK?" Millerberg is heard asking in a recording played in 2nd District Court.
"Don't do that," Aeschlimann said. "You know the answer."
Dea Millerberg, 40, is charged with desecration of a human body related to the girl's death. Her trial is scheduled for April. She was granted "use immunity" for her testimony Wednesday, meaning it can't be used against her.
Charges were never filed against Smith, as he had made a deal with police that if he led them to the girl's body, he would not be charged.
Prosecutors expect to rest their case by mid-day Friday.