The baby's tiny body was discovered in the brush along Fisher Avenue in Ocean Shores on Friday by a woman who was out walking her dogs. She found the baby after she noticed a bloody washcloth hanging from a nearby tree branch and went to investigate.
Acting on citizen tips, police identified the child's 21-year-old birth mother and her boyfriend the next day and brought them in for questioning.
An autopsy on Sunday found that the baby died "as a result of skull fractures and brain injuries due to blunt force injury of the head," said Lane Youmans, Grays Harbor County deputy coroner. "The manner of death is homicide."
Officials earlier said they would decide whether to file charges against the birth mother or her boyfriend after receiving the results of the autopsy. It was not immediately known whether the mother or her boyfriend were now under arrest.
Investigators believe the baby was born at the Oasis Motel in Ocean Shores, then brought to the Fisher Avenue location and dumped in the brush.
Diane Smith, the woman who found the baby, said the child appeared to be newly born.
"I saw part of the face and the ear and a little bit of the arm and hand ..." she says. "I can't believe anyone could do this to their own child."