Owyhee County Sheriff Perry Grant said that at about 10 a.m. Tuesday, a farm worker reported that he had found a car parked near the Hammett Bridge, not far from Bruneau.
Deputies found Desiree Vistercil, 31, of Jordan Valley, and her 16-month-old daughter dead inside the 2005 Ford Focus.
The daughter had been drowned in water in a five-gallon bucket inside the car; Vistercil died from a single gunshot to the head.
Grant said initially detectives were investigating both deaths as homicides, but now it appears to be a murder-suicide.
Grant said the incident has shaken his staff, some of whom have children.
“I want to express by my deepest condolences to the families of the deceased and my sincerest appreciation to the first responders,” Grant said in a statement on Thursday.