Eight-year-old Madyson "Maddy" Middleton died from strangulation and stab wounds to the neck, authorities said.
Coroner's officials determined the girl died of "positional asphyxiation," or strangulation, and found stab wounds on her throat, said Lt. Kelly Kent of the Santa Cruz County Sheriff's Department. Maddy's death was ruled a homicide.
Maddy's disappearance July 26 from the Santa Cruz arts apartment complex where she lived with her mother touched off a frantic search that culminated about a day later with the discovery of her body in a recycling bin on the grounds.
A neighbor, Adrian Jerry Gonzalez, 15, was arrested shortly afterward. He has been charged with one count of kidnapping and four sexual assault-related offenses.
Authorities allege the teen lured Maddy to his mother's apartment, where they were alone, and killed her. He then carried her body downstairs to a garage and hid it in a recycling bin, Santa Cruz Police Chief Kevin Vogel said.
Maddy had been tied up and sexually assaulted, according to Santa Cruz County Superior Court documents. The suspect used a weapon in the attack, according to the criminal complaint.
Police believe Maddy had been killed before they started searching for her.
Maddy was last seen about 5:05 p.m. that Sunday on surveillance video that captured her riding her scooter in the eight-acre Tannery Arts Center on River Street.
More than 24 hours after she disappeared, a Santa Cruz police detective discovered Maddy's body at the bottom of the recycling bin.
Investigators saw Gonzalez standing near the bin, Vogel said, and police believe he was watching them as they searched.
Police believe Maddy willingly went to Gonzalez's apartment because she trusted him.
If convicted, Gonzalez faces life in prison. The Santa Cruz teen is not eligible for life without parole or the death penalty because he is a minor.