Santa Cruz police late Monday said they found a young girl's body believed to be that of Madyson "Maddy" Middleton, an 8-year-old girl who went missing Sunday.
The body was found in a dumpster just before 8 p.m. inside an enclosed area in the apartment complex where the girl lived, police said.
Police also said they have arrested a 15-year-old boy who lives in the complex on suspicion of homicide.
Speaking at a press conference late Monday, Santa Cruz Police Chief Kevin Vogel said the discovery of the body was "extraordinarily heartbreaking news." Though the coroner hadn't positively identified her, in a statement police said they believed the body belong to Maddy.
Maddy was last seen about 6 p.m. Sunday riding a scooter outside her apartment complex, the Tannery Arts Center on River Street.
Witnesses said the arrested teen, who also lives at the apartment complex, was calm and was cooperating with police when he was detained just before 8 p.m. Monday.
Police did not identify the teen, as he is a minor. But Terrence Keller, a neighbor, said of the boy: "I've known him a long time. He's a nice, well-spoken kid. It's very unfortunate to say the least." A woman who appeared to be the boy's mother screamed in hysterics as her son was taken away by police.
Earlier in the day, the FBI joined the search for the missing girl. Search crews and special K-9 teams spent the day scouring the complex, a nearby levee and the beach along the Pacific Ocean.
Maddy was riding her white Razor scooter, wearing a purple knee-high dress with black leggings, helmet and long dark hair pulled to the side in a braid. The girl, standing at 4 feet tall and weighing 48 pounds, was captured on video surveillance, which Santa Cruz police shared with the public.
"She was in the courtyard where she was supposed to be," her mother, Laura Jordan told NBC affiliate KSBW. "I already fell apart. Now, I'm just in survival mode. I can't explain how difficult this is."
Jordan added that her daughter has never run away, and that she's checked with all her friends, and assisted police in knocking on every door in the area.
Police said they will hold a press conference Tuesday to release further details.
NBC Bay Area's Lisa Fernandez and Stephanie Chuang contributed to this report.