Justin Robinson admitted choking Autumn to death in the basement of his home, Faulk said. He did not offer a motive.
Autumn, a seventh grader at Clayton Middle School, was reported missing the night of Oct. 20, 2012, nine hours after she left her father's house on her white BMX bike.
After a search that involved 200 officers from 20 local, state, and federal law enforcement agencies and numerous residents, her body was found two days later about a mile away in a recycling bin next to the Robinsons' house.
Justin Robinson, a Clayton High School student who was 15 at the time of the killing, and Dante Robinson, a special-education student at the Bankbridge School in Sewell, were arrested and charged with her murder hours later.
Four days before Autumn disappeared, Justin had invited her on Facebook to his house to work on her bicycle and potentially exchange parts.
She arrived at the house at 2:45 p.m. Oct. 20. Justin's mother and stepfather were not home at the time Autumn was killed, the Prosecutor's Office said. Attempts to reach the mother, Anita Saunders, were unsuccessful Wednesday.
The case was transferred to Camden County prosecutors after Autumn's godfather, Paul Spadafora, filed civil litigation against the Gloucester County Prosecutor's Office in February. Spadafora has said Prosecutor Sean Dalton botched the search for Autumn, suggesting she might still be alive if it had been handled differently. Spadafora could not be reached Wednesday.
Dalton's office has said Autumn likely was dead before she was reported missing.
Court proceedings remained in Gloucester County.
Neighbors and family described Autumn as a bicycle enthusiast. A month after her death, the Gloucester County Board of Freeholders named a county bike trail after her.
Faulk said the case relied on circumstantial evidence.
While the evidence "strongly pointed to the fact" that Autumn was killed in the Robinson home, it would have been difficult to prove "Justin Robinson's guilt beyond a reasonable doubt without his admission in court today," Faulk said.
Had Robinson not entered the guilty plea, it also would have been difficult to move his case to adult court, Faulk said. And if he had managed to stay in the juvenile system, he would have faced a maximum sentence of 20 years, with eligibility for parole after serving one-third of the sentence, Faulk said.
"The nature of the crime demanded that Justin Robinson be sentenced as an adult facing adult penalties," he said.
Attorneys for Autumn's father, Anthony Pasquale, and her mother, Jennifer Cornwell, said their clients were pleased with the plea agreement and praised prosecutors for their efforts and consultation. The parents are divorced.
About 25 members of the Pasquale family were present in court Wednesday for the plea, Faulk said.
"Mr. Pasquale has mixed emotions about today. Obviously there was some satisfaction in having the defendant take responsibility for this tragic event, for murdering his daughter," said Doug Long, Anthony Pasquale's attorney.
Cornwell's attorney, Jaime Kaigh, said: "Every case is unique. Certain problems in this case made this resolution palatable to my client."
Autumn's death roiled the small Gloucester County town and exposed a rift in her family. In February, Cornwell filed suit against Anthony Pasquale in a dispute over control of a $100,000 memorial fund set up in Autumn's name.
Cornwell alleged that Pasquale had her name removed from the bank account controlling the fund without telling her and that she had been excluded from the decision-making process for her daughter's tombstone. They settled in March.
Justin Robinson's sentencing is scheduled for Sept. 12 before Superior Court Judge Walter Marshall Jr. Faulk said he did not know when Dante Robinson would next appear in court.
Contact Andrew Seidman at 856-779-3846, firstname.lastname@example.org or @AndrewSeidman on Twitter.