The person who killed 7-year-old Somer Thompson may have preyed on children before, making the search for the second-grader's slayer that much more urgent, police said.
Clay County authorities said they are running frustratingly low on credible leads despite receiving more than 1,000 tips about Somer's kidnapping and murder. But "America's Most Wanted" host John Walsh said county Sheriff Rick Beseler is "looking very hard at certain people.
"The sheriff and I both feel that it's a predator who has probably done it before," Walsh, who has been following Somer's case since she disappeared last week, told "Good Morning America" today.
He pointed to a string of four child disappearances in the Jacksonville area about 20 years ago that have never been solved.
"I would hate to think this is a serial child killer who has raised it's ugly head," Walsh said, noting that it's not unusual for such a predator to come out of "retirement."
Somer disappeared Oct. 19 while walking home from school with her twin brother and older sister, 10. She ran ahead after a squabble with her siblings and vanished shortly after.
Somer's body was found Wednesday in a Georgia landfill after detectives followed garbage trucks from Somer's Orange Park, Fla., hometown in search of clues.
A public visitation will be held tonight; her funeral is Tuesday.
Police say they've checked out nearly all the more than 90 sex offenders living within a three-mile radius of Somer's home and repeated searches of a vacant house near where the little girl disappeared have not yet yielded any obvious clues.
Police are hoping that samples taken from the house and the landfill where Somer's body was found will give investigators something to go on.
"All I want is to happen is that my baby didn't die in vain and we catch him," Somer's mother, Diena Thompson, said.
Walsh called Somer's killer a "cool, calculating predator" who may strike again.
"People should be on high alert there because there is a child killer at large," he said.
Indeed, neighborhood parents, many of who attended fund-raisers over the weekend for the Thompson family, are on edge, keeping a close eye on their children.
"When is this going to stop," area resident Lou Ellen McGill said. "When are they going to leave our kids alone?"
Even at the fund-raiser -- were several children were fingerprinted as a precaution -- parents looked around nervously, wondering if Somer's killer could be among them.
"I wouldn't be surprised if he was here, that's how I really feel about it," Amy Hand said.
Thompson told "Good Morning America" last week that she wanted her daughter's killer to know one thing: "We're coming for you.
"We'll get you," she said. "And, hopefully, justice will be served."
A preliminary autopsy has been completed, but Clay County Sheriff spokeswoman Mary Justino declined to provide details on the cause of death or the condition of the body.
Thompson said last week that she has been wracked by feelings of guilt and responsibility for being at work when her youngest daughter disappeared.
"I feel responsible," she said. " If I could have just, I don't know, left work or something and been able to pick her up, this wouldn't have happened."
And her surviving children are also struggling. Thompson said she's afraid to let them out of her sight, lest it happen again. They will be getting counseling after Somer's funeral, she said.
"They go through spurts, you know, they're kids," she said. "One minute they're happy, and the next minute they're sad."
A group called Justice Coalition has put up a $30,000 reward for information leading to the arrest and conviction of Somer's killer. Some of that reward money came from Crimestoppers, which urged anyone with information to call 866-845-TIPS. Callers can remain anonymous.
Clay County police asked tipsters to call 877-227-6911.