A jury convicted him Thursday of the lesser charge of second-degree murder.
On Friday, prosecutors and Lunde's mother asked Circuit Judge Ronald Ficarrotta to give him the maximum sentence for that charge, which was life.
"We're the biggest losers," said Lunde's mother, Kelly May. "We've lost Sarah."
Sarah disappeared from her home in Ruskin, south of Tampa, in April 2005. Prosecutors said Onstott strangled her after going to the trailer one night looking for her mother, whom he had dated. Her partially clothed body was found a week later in an abandoned fish pond.
Onstott apologized for any pain he had caused. "I know she don't want to hear it, Kelly, you know, my condolences," he said. "I'm sorry."
May muttered something inaudible in reply.
Onstott's attorney, John Skye, told Ficarrotta that Onstott is an attentive father and grandfather who held a job and paid child support but has made some mistakes.
"I would ask you to look forward," he said.
But Ficarrotta called Onstott a "volcano" who poses a threat to the community.
"Sarah Lunde was 13 years old," he told Onstott. "She was murdered. And you are responsible. From the evidence, it's clear that Sarah Lunde did not have an easy life, but it was her life. And you chose to end that life. And for that, you deserve the maximum punishment."
Among those in the packed courtroom was Mark Lunsford, the father of Jessica Lunsford, a 9-year-old girl who was kidnapped, raped and buried alive by a convicted sex offender north of Tampa shortly before Sarah's death.
"Another one bites the dust," he told reporters outside. Also Friday, Onstott's taped confession, ruled inadmissible in court because he was not given proper access to an attorney, was released to the media.
In it, detectives questioned Onstott about the night of the murder. Speaking softly, he struggled to remember most of what happened but admitted responsibility several times.
"I'm guilty of murder," he said at one point.
Asked if he recalled seeing Sarah, Onstott said yes. He said he had knocked on the door to the trailer and Sarah told him to open it. Inside, an argument ensued, though all he could say was that they talked about her mother.
"One thing led to another and that's what happened," Onstott said.
He remembered choking her, but could not say where. Nor could he say how Sarah's body was moved and left in a pond. He acknowledged drinking heavily that evening.
"Can you tell me the reason why maybe this happened?" one of the detectives asked.
"The sad part is, no," Onstott replied.
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