When emergency responders arrived at a home in Brooklyn Park, Minnesota, a few kilometres northwest of Minneapolis, they saw McMorris sitting on a bed as he talked on the phone.
The 7-month-old was lying on the floor, motionless.
She was pronounced dead less than two hours later.
Details in the statement of probable cause indicate that the infant died a painful death. She had 11 rib fractures, plus bruises on her scalp, abdomen, back and buttocks. Her lacerated liver bled into her abdominal cavity, the document says.
Based on her injuries, authorities said she was struck in the abdomen multiple times. They ruled her death a homicide.
Hennepin County Attorney Mike Freeman told the Washington Post that McMorris killed the child after finding out he wasn't her biological father. A paternity test confirmed as much just three days before the child was killed.
McMorris, 24, was charged last week with murder, punishable by up to 30 years in prison.
"I've been in this business for 18 years as a prosecutor," Freeman told the Post. "If you don't get upset by cases like this, you should get out of the business."
When police arrived at the home in March, McMorris was unable to explain why the baby had bruises or why she suddenly stopped breathing, according to the statement of probable cause. He only said that the child was vomiting a white substance.
McMorris' lawyer, Ira Whitlock, told the Post that his client is innocent and that there were inconsistencies in the accusations against him.
The child's mother, Whitlock said, told investigators that she didn't notice the bruises and scratches that the day-care workers had spotted. According to the probable-cause statement, the mother was unable to explain why her daughter had several injuries. She said the child had a cold but was otherwise healthy and was acting normally - playing, crawling and eating - the night before she was found dead.
"Just because a person ends up being the last person with the baby when the baby dies doesn't mean that person caused the injuries to the baby," Whitlock said. "Mr McMorris was with the baby, but it doesn't mean he inflicted the injuries to the child."
Neither the mother nor the infant is named in the charging documents. The mother was living with McMorris and another child, a 12-year-old relative.
Just because a person ends up being the last person with the baby when the baby dies doesn't mean that person caused the injuries to the baby
McMorris is in custody in the Hennepin County Jail and is scheduled for a court hearing on September 22.
The 7-month-old's death occurred in the same county where Cory Morris stands accused of killing his 4-month-old daughter, Emersyn.
Morris punched the infant about 15 times to quiet her down, authorities said. Morris was charged with murder last week - just days before McMorris was charged with the same crime.
According to the Centres for Disease Control and Prevention, state and local child protective services in 2012 received about 3.4 million reports of children being abused or neglected. The majority of them, 78 per cent, were victims of neglect and 18 per cent suffered physical abuse, according to the CDC.
About 80 per cent of the perpetrators were parents, the CDC said; 4 per cent of perpetrators were the parents' unmarried partners.
