Police arrested 40-year-old Jessica Smith on Sunday.
Her youngest daughter was found dead at a resort in Cannon Beach, Oregon, on Friday.
Smith’s 13-year-old daughter was badly hurt; she was taken to a hospital in critical condition.
Police say smith was the only other person in the room when the two were attacked. As such, they say she’s their only suspect.
A preliminary autopsy found Smith’s toddler died as a result of homicide, though the cause of death is pending a toxicology report.
Smith’s teenage daughter is in stable condition at a hospital near Portland, where she is in protective custody.