A sex offender charged in the slaying of a 13-year-old girl pleaded not guilty Friday, and prosecutors said they plan to seek the death penalty.

An attorney for David Lee Onstott (search) entered the plea to charges of murder and attempted sexual battery.

Meanwhile, prosecutor Mark Ober filed court papers Friday saying he will seek the death penalty for Onstott if he is convicted of first-degree murder.

The massive, weeklong search for Sarah Lunde (search) captured the nation's attention last month before her partially clothed body was found April 16 in an abandoned fish pond near her family's mobile home in Ruskin. She was last seen April 9 after returning home from a church trip.

Authorities said Onstott, 36, confessed to strangling Sarah after showing up at the house early the next morning looking for the girl's mother, Kelly May, whom he had once dated. He allegedly told investigators that he killed Sarah after the two got into an argument.

Onstott 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.