Bruises and burst blood vessels on her head and neck indicated Lieba had tried to strangle the girl, according to law enforcement officers and doctors who examined her.
The sexual assault injured the girl and infected her with an incurable sexually transmitted disease, said Dr. Cynthia Burrows of the Billings Clinic who examined the girl and testified at Lieba's April trial.
"The child will forever be reminded of this trauma," Assistant US Attorney Ryan Weldon wrote in a sentencing memo to U.S. District Judge Brian Morris. "The United States without hesitation recommends life in prison. Lieba, through his actions, forced every parent in Montana to confront their worst nightmare."
During Lieba's three-day trial, a 12-year-old neighbour of the victim testified that she also had been chased by Lieba at a playground in the town of Wolf Point at around 9.30pm on the night the toddler went missing, but she managed to escape.
The defendant's attorneys said he had stopped taking his anti-psychotic medication prior to the kidnapping and could not remember the events of that night.
When he was initially questioned by police after his arrest, Lieba said he had no idea where the victim was, and when asked if she was dead, he replied, "probably," reported the station KRTV in April. He also told the detectives to "go get the body."
Defense attorneys requested 30 years in prison, noting that Lieba would be approaching middle age by the time of his release.
"The most effective protection for the public will be for Mr. Leiba to address his substance abuse and mental health issues," Assistant Federal Defender R. Henry Branom wrote in documents.
Both Lieba and the victim are American Indians. The Fort Peck Reservation is home to the Assiniboine and Sioux tribes and has a population of about 10,000.
The kidnapping and assault of the girl was followed weeks later by the killing of a 1-year-old girl by a caregiver who dumped the victim's body in a trash can.
Tribal officials have blamed both crimes on the rising use of methamphetamine on the reservation, but it's uncertain what involvement Lieba had with the drug.