A next-door neighbor of a Northern California couple accused of chaining up and torturing a teenage boy for more than a year was deeply involved in the abuse, authorities say in court documents unsealed Wednesday.

The neighbor, 29-year-old Anthony Waiters, was the last of four people charged in a case that has horrified the California town of Tracy since December, when the boy escaped from his captors and showed up bruised, emaciated and wearing only boxer shorts at a local gym.

Waiters' connection to the alleged abuse had been unclear until the documents were unsealed Wednesday in San Joaquin County Superior Court.

According to one search warrant affidavit, the teenager implicated Waiters and told detectives that he was "an active participant in his torture."

The 16-year-old wandered into a fitness center on Dec. 1, wearing a chain padlocked to his bloodied ankle and covered in soot. He told police he had been shackled to a fireplace grate, beaten in the head with a baseball bat, choked with a belt and denied food for days at a time.

Three people were arrested by the following day: Michael Schumacher, 34, his wife, 30-year-old Kelly Lau, a Girl Scout leader, and Caren Ramirez, 43, the boy's former guardian. All three have been charged with torture, kidnapping and multiple child abuse charges. Ramirez had been on probation for a previous case of felony child abuse related to the teenager.

Waiters, who was arrested a week after the others, is charged with child abuse, aggravated mayhem, false imprisonment and child endangerment. All four defendants have yet to enter pleas and are awaiting a Feb. 17 arraignment.

The court documents also show that authorities are probing MySpace accounts that detectives say Waiters and Schumacher used to communicate with each other.

Schumacher, who owned the two-story, Tudor-style house where police say the teen was held, posted a photo of himself and Waiters on his MySpace profile, according to the documents.

Detectives have seized cell phones, cameras, computer equipment and DVDs found at the house to determine whether the teen's captors documented any of the abuse, search warrants unsealed Wednesday show. The filings do not say what was found on the devices or the DVDs.

Child abuse researchers said the case was unusual because the abuse alleged in the court documents involved a neighbor.

"Thankfully, this kind of case is relatively rare, in which you have strangers to the child recruiting complete outsiders to participate in this level of torture," said Murray Straus, co-director of the Family Research Laboratory at the University of New Hampshire. "It's very hard to understand a motive."

A previously released affidavit said the boy told a detective he had been forced to drink alcohol, smoke marijuana and take pills to keep him lethargic. The court documents say detectives found marijuana, smoking pipes, and cold and allergy medicine inside the house.

They also found an aluminum baseball bat near the fireplace, according to the documents released Wednesday.

Authorities have not described Ramirez's relationship to Schumacher and Lau. Neighbors reported seeing the woman at the couple's home, but it was unclear how they first came into contact.

The documents released Wednesday shed no light on their connection but revealed that Ramirez fled to a friend's house in Berkeley the night the teen escaped.

Police previously said Ramirez became the teen's guardian after he was taken from an abusive father three or four years ago. The boy later reported that she had beaten him with martial arts sticks, a broomstick and clothes hanger, prompting police to arrest her. Court documents show Ramirez pleaded no contest to one felony count of beating the boy.

The teen was then placed in a group foster home in Sacramento but ran away in May 2007. He told police he then was "kidnapped" while walking along a highway by two men who might have had "Spanish-speaking accents," according to an affidavit included in the court documents released Wednesday.

The documents did not indicate what happened next or how the teenager came to live with Schumacher and Lau. They also do not suggest a possible motive for the physical abuse and captivity.

Police and prosecutors declined to answer questions about the case Wednesday.

Attorneys for Schumacher, Lau and Ramirez did not immediately return calls seeking comment. A clerk in the office of Waiters' attorney said the lawyer was prohibited from speaking about the case.