The additional charges were announced against Gary Hilton by Lee Darragh, the district attorney in Dawson County, where Meredith Emerson's body was found Monday night. Mr. Darragh said the 24-year-old died Friday from a blow to the head, three days after she disappeared during a New Year's Day hike in the north Georgia mountains.
Mr. Hilton, 61, was charged Saturday with kidnapping with intent of bodily injury. He is being held in the Dawson County jail on the additional charges.
The agreement with Mr. Hilton covers only the prosecution for the Emerson case. Other jurisdictions in Georgia and other states would be free to seek the death penalty for other killings if they find connections, Union County District Attorney Stan Gunter told The Associated Press.
U.S. Attorney David Nahmias, who was interested in prosecuting the case as a federal death penalty case because the kidnapping is believed to have occurred on U.S. Forest Service land, agreed to the deal if Mr. Hilton cooperated, Mr. Gunter said.
"He agreed that finding the body of Ms. Emerson was very important, and he agreed if that was the resolution, then that would be a just resolution," Mr. Gunter said. Mr. Gunter said authorities had little hope of finding Ms. Emerson's remains without Mr. Hilton's help.
Asked whether the family was alerted to the arrangement with Mr. Hilton before it was made, family spokeswoman Peggy Bailey said, "We're just not discussing anything about that at all."
Other jurisdictions, including officials in North Carolina and Florida, have expressed interest in trying to determine whether the Emerson case is connected to killings there.
Mr. Hilton appeared Monday before a judge who denied his request for bail.
Hours later, he led investigators to a spot in a wooded area in north Georgia where they found Ms. Emerson's body, said John Cagle, the special agent in charge of the Georgia Bureau of Investigation's Field Division in Cleveland, Ga.
An autopsy showed Tuesday that Ms. Emerson died of a blow to the head.
Kris Sperry, the state's chief medical examiner, found that after Ms. Emerson received the lethal injuries, she was decapitated, Georgia Bureau of Investigation spokesman John Bankhead said. He declined to discuss any other details of the autopsy.
Authorities have said Mr. Hilton could face more charges.