An autopsy concluded that the cause of death was blunt force injuries to the dead, Themas wrote.
According to the criminal complaint, doctors stated the injuries the infant suffered were not the result of an accidental fall.
After a 14-month-long investigation, police determined that the infant's injuries were allegedly "non-accidental."
Marxen has been charged with first-degree murder, child endangerment resulting in death, and child endangerment and is being held in Scott County Jail on no bond.
A 2019 Des Moines Register investigation found that there have been numerous child deaths reported at day cares where authorities had previously warned providers they were caring for too many children, often in unregulated facilities. Authorities have found drugs in some central Iowa day care facilities and found people convicted of felonies who were living in houses operating as in-home day cares.