SAN FRANCISCO — A Girl Scout leader and her husband were arrested after an emaciated, terrified and nearly naked 17-year-old showed up at a gym with a chain locked to his ankle, saying he had just fled his captors, authorities said Tuesday.

Police were also seeking the boy’s aunt and had an outstanding felony warrant against her for an earlier alleged assault against the teen.

The boy, who authorities said ran away from a Sacramento foster home last year, came into the In-Shape Sports Club fitness center Monday wearing only boxer briefs and covered in what appeared to be soot, said gym manager Chuck Ellis. The teen was scared someone was going to come after him and asked to be hidden, Ellis said.

“He said, ‘Don’t let them get me, don’t let them get me,’ ” Ellis said. “He was totally terrified.”

The boy said he had been held captive for nearly a year, said Ellis, adding that he looked as if he were only 10 to 12 years old.

Authorities said they believe the boy had been chained to a car seat but picked up a dropped key, unlocked himself and fled when the car stopped.

Police arrested Kelly Layne Lau, 30, and Michael Schumacher, 34, late Monday after questioning the couple, and a subsequent search of their nearby home found further evidence implicating them, Tracy police spokesman Matt Robinson said.

They were booked on charges of torture, kidnapping and child abuse, and were set to appear in court Thursday, according to online jail records. The couple were being held at San Joaquin County jail on bail of nearly $1.2 million each.

Lau and Schumacher’s four young children, two of whom were home when police arrived, have been taken into protective custody, authorities said.

Lau started serving as a local Girl Scout leader sometime in September, after a background check turned up nothing to cause concern, said Pam Saltenberger, chief executive of Girl Scouts Heart of Central California.

Police still were seeking Caren Ramirez, 43, the boy’s aunt, on suspicion of participating in the alleged abuse, Robinson said. Investigators were trying to figure out the connection between the couple and Ramirez, who authorities believe occasionally visited the home in Tracy, about 70 miles south of Sacramento.

Ramirez had become the teen’s guardian after child-welfare officials took him from his abusive father three or four years ago, Robinson said. After Ramirez was arrested for allegedly abusing the boy, he was placed in another foster home, which he fled in late 2007, police said.

Since then, the boy’s whereabouts hadn’t been known until around 4 p.m. Monday, when he entered the fitness center, Robinson said.

“The victim says he was held against his will,” Robinson said. “When you have a 17-year-old boy showing up with a bloody ankle from having a chain wrapped around it, it’s one of those things for officers to put one and one together.”