Police say Adrian Jerry Gonzalez lured Madyson Middleton from a courtyard where she had been riding her scooter over the weekend into his family's apartment where he attacked and killed her.
Gonzalez remains in custody at the Santa Cruz County juvenile detention center, where he has been held since being arrested Monday evening near a recycling bin where the girl's body had been dumped.
"Unfortunately, the search for Maddy ended in the worst way possible," Santa Cruz District Attorney Jeffrey Rosell said.
He said Gonzalez will be charged as an adult and added that in his two decades in Santa Cruz, he's never seen a 15-year-old charged with murder.
Prosecutors said they considered the nature of the crime, how the crimes were committed, the age of the defendant and prior criminal record when making their charging decision.
Because of his age, authorities have declined to say whether Gonzalez had a record.
Authorities also say they don't have a clear motive in the killing. "People do things for lots of reasons, sometimes we understand it, sometimes we don't," Rosell said.
A call to the Santa Cruz County Office of the Public Defender for comment on the case wasn't immediately returned. Gonzalez could be assigned a public defender at an upcoming court appearance.
Neighbors at the Tannery Arts Center where both the suspect and Madyson lived said they were stunned by the death. The center is a public-private nonprofit that includes 100 affordable loft apartments for artists and their families. About 250 people live in the complex, including about 50 children.
A memorial has been set up in Madyson's honor, overflowing with dozens of bouquets, stuffed animals, balloons, candles and notes.
On Wednesday, a group of girls about Madyson's age were crying at the memorial. One girl was walking Madyson's dog, Lucy.