Robinson said 43-year-old Ramirez, who is the boy's aunt and has a felony record, had been on the lam since being charged with abusing the same teen in an unrelated incident.
"At one point [Ramirez] was his guardian after the juvenile was taken away from his parents," Robinson said. "She eventually was arrested for assaulting the juvenile herself. She had an outstanding warrant because of that arrest and so for the past year and a half she has essentially been on the run trying to flee that arrest warrant."
"She will eventually be brought back to Tracy for questioning and eventually arraignment," Robinson said.
Ramirez's arrest follows the apprehension of 30-year-old Kelly Lau Schumacher and her 34-year-old husband, Michael Schumacher, who were arrested and charged with suspicion of torture, kidnapping and child abuse.
The Schumachers, who lived only blocks from the fitness center where the boy was found, each have their bail set at nearly $1.2 million.
Robinson said police are still trying to determine the connection between the Schumachers and Ramirez.
The Back Story
The peculiar tale began unfolding Monday when a teenage boy arrived at the In-Shape City Sports Club around 4 p.m. begging an employee to hide him, according to Robinson.
"The staff noticed blood and bruising on his body, and he looked emaciated and had a chain locked to his ankle," Robinson said. "The boy was confused and didn't know where he was or how long he had been held against his will."
Chuck Ellis, the district manager for In-Shape Health Clubs, was one of the first people to come to the teen's aid. Some witnesses said the boy had jumped over a fence to get to the fitness center, according to local reports.
"It was rather shocking. I mean you couldn't believe when we saw him, he was very thin, very small in stature for when we found out what his age was, it was very surprising," Ellis said. "We thought he was 10 or 13 years old."
Ellis said the teen pleaded with staff to hide him, worried that his captors would come after him.
"You could tell that he was injured. He had cuts on his arms, on his back. [He was] totally terrified. The look in his eyes was just unbelievable. I've never seen [anything like that,]" Ellis said. "He was covered in dirt, kind of like a soot that was all over him."
"I was just horrified how he looked with the cuts and everything but then to see a chain wrapped around his ankle," Ellis said. "It was unbelievable."
Foster Teen Abuse
Ellis said the teenager told him that he was picked up after running away from the foster home to look for his family and that he was held for almost a year.
Police said the boy's left ankle was padlocked with a chain and his leg appeared to be cut. He didn't know where he was but knew his name and birthday.
They took the boy to Sutter Tracy Community Hospital, where he was treated for cuts and "other injuries," some of which may have been from sexual abuse, according to police.
"We want to make sure he gets the proper medical attention necessary," said Robinson.
Hospital spokeswoman Karen Mudd would not provide any other details to ABCNews.com and said the boy was being held in "protective services."
After interviewing the boy, police determined he had been kept at a house "down the street" and knocked on the door of the Schumachers, interviewing the couple and arresting them. By midnight, police had obtained a search warrant for the house and pressed charges.
Police continue to interview Kelly Lau-Schumacher, while her husband is in custody at the San Joaquin County jail. The couple will appear in court Dec. 4.
Meanwhile, four children - ages, 1, 3, 11 and 9 - were also taken from the house, according to police.
Police said the boy had previously been reported missing from a group foster home in Sacramento since 2007, but they had no idea how long he had been there or where his family was.
An unidentified neighbor told the Tri-Valley Herald that he had seen the boy playing with other children and thought he was a family member. The neighbor said he had limited contact with the couple, who had lived in the house for about a year.
"They appeared to be a normal family," the neighbor said. "We don't even know their last name. [The boy] wouldn't be allowed to talk to us when we'd see him outside or ask him questions."
Just this week Tracy, a city of about 80,000 one hour north of San Francisco, was named by CQ Press as the second-safest city in Northern California and the 46th-safest city in the nation.
Robinson said he had never seen a case like this in Tracy. "It's really abnormal here," he said.