The convicted sex offender accused of killing a 13-year-old girl, then dumping her body in a pond was caught trying to dig out of his jail cell, officials said.

David Onstott (search), 36, was found scratching on the concrete wall near the floor of his cell early Saturday with a metal towel holder he had apparently pried off a desk, said Hillsborough County (search) sheriff's spokeswoman Debbie Carter.

He had only scraped about a quarter of an inch into the concrete, she said. He was later moved to another cell in the jail.

Police said Onstott confessed to choking Sarah Lunde (search) after getting into an argument with the girl at her Ruskin home in April. Her partially clothed body was found a week after she disappeared in an abandoned fish pond about a half-mile from her home.

Onstott is charged with first-degree murder and attempted sexual battery. He has pleaded not guilty. He served 5 1/2 years in prison for a 1995 rape and was arrested in March on a charge of failing to register as a sex offender.

Prosecutors have said they will seek the death penalty.