RALEIGH, N.C. — An aspiring chef slain over the summer at a North Carolina mountain inn died during a sexual assault by a co-worker, prosecutors said Thursday, offering new details on the motive for the killing at the height of tourism season along a famed scenic highway.

New charges filed this week in federal court could also allow authorities to seek the death penalty in the slaying that took place steps from the Blue Ridge Parkway.

The new superseding indictment adds two counts of aggravated sexual abuse resulting in death against 21-year-old Derek Shawn Pendergraft, and it charges him with first-degree murder. He was previously charged with second-degree murder, which would have carried a maximum of life in prison.

Prosecutors said in a news release that they haven't yet decided whether to pursue a death sentence.

The indictment said that Pendergraft killed Sara Ellis on July 24 during the course of a sexual assault. Prosecutors write that Ellis was "particularly vulnerable due to her ... hearing impairment." A cause of death hasn't been released.

A defense attorney didn't immediately respond to a message seeking comment Thursday.

Pendergraft had initially told authorities the two went for a hike near the Pisgah Inn where both worked, and that he lost track of Ellis after she turned back in the rain. After he informed inn managers she was missing, authorities found the victim's body off an embankment near the parkway.

Family and friends said that Ellis, 29, was an aspiring chef from Florida who loved to bake and watch cooking shows. She saw the seasonal job at the inn as a way to advance a career in restaurants, they said.

A close friend of Ellis, Nicole de Giovine, said Thursday that the death has been crushing for family and friends — especially Sara's twin sister, Rachel, who has coped by limiting her exposure to developments in the criminal probe.

"Sara's death has been devastating to her, and she is not ready to know the details of the case," de Giovine said in an email, adding that knowing her sister was sexually assaulted would "only devastate her further."

The death also rattled a group of young, seasonal employees who had come from around the country to work at the inn operated under contract with the National Park Service, its owner Bruce O'Connell said over the summer.

He said about half of the inn's 100 employees live on-site in dorm-style rooms at the property, which is about 30 minutes from surrounding towns. Pendergraft, who's from Asheville about 45 minutes northeast, had been working as a housekeeper at the inn.