Ruskin, Fla. ? A registered sex offender confessed to killing a 13-year-old girl who disappeared a week ago, saying he got into an argument with her and he choked her to death in her home, the sheriff said Sunday.

David Onstott, 36, was charged with first-degree murder Sunday, a day after investigators found Sarah Lunde’s partially clothed body in an abandoned fish pond, Hillsborough County Sheriff David Gee said.

Sarah was last seen April 9, shortly after returning home from a church trip. Early the next morning, Onstott paid an unexpected visit to the family’s home to look for Sarah’s mother, Kelly May Lunde, whom he once dated, Gee said.

After Sarah let Onstott into the house, they got into an argument and Onstott put her in a choke hold and killed her, Gee said.

Sarah’s 17-year-old brother came home later and found the front door wide open and his sister gone, but the family initially assumed Sarah had gone to a friend’s house. She was not reported missing until April 11.

Onstott, who spent 5 1/2 years in prison after being convicted in 1995 of raping an adult acquaintance, has been held without bail in the Hillsborough County Jail since Tuesday on unrelated charges. His attorney, Pat Courtney, declined comment Sunday.

Sarah’s relatives and members of her First Apostolic Church congregation turned out in droves Sunday to tearfully mourn the loss of the girl. Her young friends dropped to their knees and wept.

Sarah’s mother was too shaken to talk Sunday, but her brother Larry May said: “It’s devastating, it’s just unbelievable.”

Pastor Johnny Cook, left, leads members of his church in prayer during a service for 13-year-old Sarah Lunde at the First Apostolic Church in Ruskin, Fla. Lunde's body was recovered Saturday from a pond near her home. A convicted sex offender on Sunday confessed to killing Sarah.

“Everybody has things they wished they’d done — spending more time with their children or keeping in closer contact,” May said.

Among the mourners were Mark Lunsford, whose daughter Jessica was found dead last month after she was kidnapped from their Citrus County home, and Roy Brown, whose daughter Amanda was murdered in 1997 by a convicted child molester in Tampa.

Both men had lent their support to the Lunde family this past week, and Lunsford had helped search for Sarah, saying some of her family had helped search for his daughter in February.

“It’s sad that it takes something like this to bring a community together,” Lunsford said. “America needs to wake up. The next child could be yours.”