BLAIRSVILLE, Georgia (CNN) -- A young hiker whose body was found in northern Georgia was probably alive for three days before she died from blunt force trauma to the head, authorities said Tuesday.

Meredith Emerson, 24, was decapitated after her death, an autopsy also found.

Investigators believe she was killed January 4, said Lee Darragh, district attorney for Hall and Dawson counties.

That's the same day suspect Gary Michael Hilton, 61, allegedly made a phone call from a pay phone at a convenience store in Cumming, Georgia, where Emerson's black leather wallet containing her identification cards was discovered in a trash bin.

The bin is next to a grocery store parking lot where Ella, Emerson's Labrador mix, was found wandering Friday, according to a criminal warrant.

Charges against Hilton were upgraded to murder Tuesday.

He led investigators to her body Monday in exchange for prosecutors' agreeing to drop consideration of the death penalty against him, according to Union County District Attorney Stan Gunter.

Hilton will be charged with one count of malice murder and is set to appear Wednesday in court in Dawson County, Darragh said.

The medical examiner has informed Emerson's family about the details of her death.

State and federal authorities said they are also trying to determine whether Hilton may have been involved in at least three other killings in two neighboring states.

Emerson, a University of Georgia alumna, disappeared on New Year's Day after leaving home for a hike in the mountains with her dog, Ella.

Investigators found Emerson's body about 7:30 p.m. Monday, in the 25,000-acre Dawson Forest Wildlife Management Area, about 30 miles south of the Union County, Georgia, state park where she went hiking, said investigator John Cagle of the Georgia Bureau of Investigation. Watch Cagle describe how the body was found »

When asked whether Hilton, 61, had taken investigators to the location or merely described it, Cagle said, "Both."

Several witnesses told authorities they saw Hilton and Emerson together, letting their dogs play along a hiking trail in Vogel State Park. Witnesses later said they had seen a van that looked like Hilton's in the area where the body eventually was found, and the search shifted there, Cagle said.

Director Vernon Keenan of the Georgia Bureau of Investigation and other law enforcement officials on Monday night withheld virtually all details about what Hilton said to authorities and the scene where they found Emerson's body.

"There's a lot going on in the case right now -- the investigation is still going on," said Union County District Attorney Stan Gunter. "A lot of decisions have to be made."

As they grieved, Emerson's family members also expressed some relief that her body was discovered.

"We are glad she's found. This brings some sort of closure for us," Peggy Bailey, Emerson's godmother and spokeswoman for the family, said, according to CNN affiliate WXIA.

Hilton was jailed in Union County on Monday night.

Several hours earlier, he appeared in court in shackles and an orange prison jumpsuit. He did not speak as Judge Johnie Garmon read the charges against him. Garmon said surveillance video had identified Hilton as he tried to use one of Emerson's credit cards at a bank in Canton, Georgia.

Authorities arrested Hilton after finding bloodstained clothes consistent with what Emerson had been wearing when she went missing, according to a criminal warrant filed Saturday.
Follow the case on an interactive map »

When Hilton's 2001 minivan was searched, agents "determined that the rear seatbelt had been cut out," the warrant said. "Hilton was attempting to vacuum the vehicle and wash portions of it with a bleach and water solution."

Hilton was taken into custody Friday at a convenience store in suburban Atlanta.

Also Monday, federal, Georgia and North Carolina investigators met in nearby Cleveland, Georgia, to compare notes on the Emerson case and a similar case from a few months ago in North Carolina, according to FBI Special Agent Greg Jones.

In the North Carolina case, an elderly couple -- John and Irene Bryant -- disappeared after going for a hike in Mount Pisgah National Forest. Irene Bryant's body was found near the couple's car. John Bryant's body has not been found, but Keenan referred to the case Monday night as a "double homicide."

Investigators have a bank video of a man wearing a yellow jacket -- believed to have belonged to John Bryant -- while using the Bryants' ATM card.

Witnesses who saw Hilton on the trail with Emerson on the day she disappeared said he was wearing a yellow jacket.

On Monday night, Keenan said Georgia authorities also plan to meet with law enforcement officials from Florida to share details about a similar killing there.