"There is probably someone out there who is very surprised the body was found so early,'' Mosquera said, noting that agents spent hours Saturday night gathering evidence at the scene near a radio tower, more than a mile off the main road.
Agents believe the killer may be familiar with the area since that dead end spur off the main road is known as an out-of-the-way spot to teens and others.
The body, which remained at the scene through much of the initial phase of the investigation, was moved to the Berkeley County Coroner's office Sunday morning.
"We owe it to little Jessie to do everything we can to get this person,'' Mosquera said. "I feel confident we will have a successful closure.''
Agents thought they had a break early Sunday, according to FBI Agent Dennis Lormel. But unfortunately, it didn't pan out. Lormel wouldn't elaborate on the nature of that information.
At daybreak Sunday, about 40 FBI agents - some of whom had been up all night - traded the cramped West Virginia State Police barracks command post for the Department of Health and Human Resources office building in the nearby Mid-Atlantic Industrial Park.
The briefing, which wasn't open to the media, was meant to bring all investigators up to the speed on the events overnight, Mosquera said.
"We will be doing more interviews (Sunday) and continuing to conduct the crime scene,'' Mosquera said.
"We're hoping to hear from anyone who saw anything or knows anything,'' Mosquera said. The number to call is 1-304-267-0000.
Mosquera said authorities had also not determined how long the body had been there when it was found.
Killeen said the rapid and massive reaction by the FBI in this case is not unusual.
"The director of the FBI (Louis Freeh) has put an emphasis on responding to missing child cases," he explained.
Among those responding were, according to Killeen, at least two members of the Behavioral Science Unit from the FBI's training center in Quantico, Va. One of their areas of expertise is profiling the type of suspect that would commit certain crimes, such as the murder of a child.
As for any possible connection to the murders three children in Spotsylvania County, Va., over the past two years, Killeen said, "It would be premature to comment on any possible links between this case and that of other missing children."
Killeen said the bureau would devote a substantial number of agents to case "until all the logical leads have been exhausted."
Berkeley County Prosecuting Attorney Pamela Games-Neely has been prosecuting cases here since 1989.
"Cases like these are very rare," she remarked, although a man was arrested several months ago for allegedly kidnapping a girl in Berkeley County. That child, however, was found alive and the man's case is still pending.
The youngster disappeared Thursday evening around 8:15 p.m. from Pikeside Bowl, on U.S. 11 south of Martinsburg, where she had gone with her father, David Newell of Baker Heights.
Jessica had been on her way to the bowling alley's game room, and was last seen at the front door waiting for an uncle who was bringing some baseball cards, according to state police.
Her disappearance launched a massive search that initially included more than 50 FBI agents, local police, family members and volunteers.
Authorities used helicopters and bloodhounds in the search that continued throughout Friday night and Saturday.
Authorities studied a security tape from the bowling alley, but turned up nothing helpful, Mosquera said.
Jessica, a second-grade student at Berkeley Heights Elementary School, was 4 feet tall, 60 pounds, with blonde hair that flowed midway down her back. She was wearing a red and pink flowered one-piece dress when she disappeared Thursday.
The body was dressed similarly when found, Mosquera said.