David Onstott, 36, was ordered held without bond at an arraignment Monday. He was charged with first-degree murder Sunday, after the registered sex offender allegedly confessed to killing the girl, saying he got into an argument with her and he choked her to death in her home.
Over the weekend, deputies found Sarah Lunde's partially-clothed body floating in a pond near the child's home, reports CBS News Correspondent Jim Acosta.
During the arraignment, Onstott did not speak.
"You don't have to make any plea at all," Judge Walter Heinrich told him. "But understand very clearly, make absolutely no mistakes: If you plead guilty today, you're going to be sentenced to jail time. The only question: how long in jail I'm going to give you."
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, Hillsborough County Sheriff David 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.
"You are talking about a person who would murder a child. Who knows what's in his mind," Gee said.
"I think that there are only two people that really know what happened during those early morning hours, and one of them can't tell us any more," Gee said on CBS News' The Early Show. "So we're getting his side of the story and I think we're going to have to wait for the forensic evidence to help us make a better determination of what actually happened."
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.
Gee said Onstott "went to great effort to keep her body from being discovered."
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.
Another young Tampa-area girl,, allegedly was murdered by a convicted child molester last month.
And CBS' Acosta reports that convicted sex offenders are four times more likely to repeat their crimes than non-sex offenders. Victims' families say registering these criminals on state Web sites is not nearly enough to prevent a criminal act from happening again.
"For many kinds of sex offenders there really is no such thing as rehabilitation for them," University of Louisville criminologist Richard Tewksberry told Acosta.
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.
"It's been terrible," sobbed Leslie Fontana, Sarah's best friend.
Sarah's mother was too shaken to talk Sunday, but her brother Larry May said: "It's devastating, it's just unbelievable."
"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, Jessica's father, 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."
But there's no easy solution to the abundance of state-registered sex offenders. Moreover, state legislation would be required before offenders could be monitored more closely.
"There's way too many sex offenders to realistically expect us to monitor them all carefully and closely," Tewksberry told Acosta.
Church members said that on any other Sunday, Sarah would have spent the afternoon walking with other church members to nearby homes to hand out Bibles.
The church had become her refuge from troubles in her life, including times when she had run away from home.
"People asked me why did she come here, why did she spend her time here," said Matt Fontana, the youth minister at the church. "Because she found love here ... Now she's in heaven."
The small church normally draws only about 50 congregants for morning services, but its pews were filled with more than double that Sunday.
Sarah had started coming to the church three years ago on her own and no other members of her family attended with her, church leaders said. She often would call two or three families to arrange for a ride and they'd all show up to get her.
"Every Sunday we talked about who was going to pick up Sarah," Sherry Cook said. "I can't believe we're not picking her up this morning."