TAMPA, Fla. -- A convicted sex offender on trial in the slaying of a Tampa-area teen three years ago confessed to the killing in a jailhouse conversation with his mother, prosecutors said during opening arguments Wednesday.

Prosecutors said jurors will get to hear the taped conversation in which David Lee Onstott whispers "I killed her" to his mother. Onstott, 40, is charged with attempted sexual battery and first-degree murder in the 2005 death of Sarah Lunde, 13.

Onstott's attorney said the tape is nearly inaudible and disputes that his client made the statement.

Prosecutor Sean Keefe said Onstott came to the house looking for Sarah's mother, whom he had dated. She wasn't there and he ended up strangling the teen, the prosecutor said. Her body, weighted down with concrete, was found a week later in an abandoned fish pond near her home in Ruskin, south of Tampa.

With no physical or forensic evidence linking Onstott to the teen's slaying, prosecutors are left with his statements and other circumstantial evidence. One piece of evidence -- a beer bottle prosecutors said links Onstott to the killing -- has never been found.

Keefe said once Onstott was in custody, he made statements to various people admitting his guilt. Onstott's attorney is expected to attack those statements as vague and unreliable.

Onstott's attorney John Skye also told the jury that there were inconsistencies in statements from Sarah's brother, Andrew, and a friend who last saw Sarah alive when they left the house early on a Sunday morning to pick up food.

"The evidence is going to show that she left when her brother didn't come back with food ... and that somebody did commit homicide on her and it had nothing to do with David Onstott," Skye said.

Maybe more significant is what the jury won't hear.

A taped confession Onstott gave to investigators was thrown out by a judge last year because he wasn't given proper access to an attorney.

Prosecutors backed off seeking the death penalty for Onstott after the judge threw out his taped confession as evidence last year. If he's convicted of first-degree murder, he would be sentenced to life in prison.

He was convicted of sexual battery in 1995, but that won't be revealed to the jury during the trial, which is expected to continue into next week.

Sarah's disappearance and the discovery of her body on April 16, 2005, attracted the attention of the nation, coming on the heels of the high-profile rape and slaying of 9-year-old Jessica Lunsford by convicted sex offender John Couey in Homosassa, Fla., north of Tampa. Couey was convicted and sentenced to death last year.