(CNN) -- The news was devastating, the aftermath surreal.

Somer Thompson, 7, went missing Monday on her way home from school.

Diena Thompson clutched a tissue in her right hand, frequently wiping her eyes as she told reporters that she couldn't believe it was her daughter whose body was found in a Georgia landfill. Her visage revealed the weight of grief and a yearning for justice.

Standing outside her home in Orange Park, Florida, Thompson looked into the camera and, with her voice cracking, sent out a stern warning to her daughter's killer.

"I want you to know that I will not sleep until this person is found. I hope they get you and I hope they make you pay for a long, long time. You don't take from somebody. You didn't just take her from me. You took her from my family, you took her from all these people. And you don't do this to a little baby and put my baby in the trash like she's nothing. That's not OK, this is not OK." Watch Thompson warn her daughter's killer »

The body of her daughter, 7-year-old Somer Thompson, was found in a south Georgia landfill, Clay County, Florida, Sheriff Rick Beseler said Thursday morning.

Nancy Grace The latest on the investigation into the death of Somer Thompson

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Authorities on Friday were combing were through a "tremendous mountain" of garbage at the landfill, a sheriff's spokeswoman said.

Some possible evidence has been removed from the landfill, but authorities won't describe what they found, said Mary Justino of the Clay County Sheriff's Department. Police have no suspects, she said.

She said other law enforcement teams are focusing on the locale where the child apparently was last seen. Justino said witnesses, including some children, saw her on the sidewalk in front of a vacant house that is being renovated after a fire.

"Since Monday, we have been trying to figure out who frequents that area," including anyone working on the home, she said.

Authorities are treating the death as a homicide.

Somer became separated from her siblings and schoolmates on her way home from school in Orange Park, Florida, on Monday.

She was seen in a fight at school before she disappeared, according to a police report released Thursday. Her 10-year-old sister told police that Somer had gotten into a fight with another girl at school earlier in the day.

The sister said she brought up the fight while she and her brother walked Somer home from school, and that Somer ran off from them, apparently upset. The sister said she lost sight of Somer in a group of other kids leaving the school, according to the police report.

The medical examiner in Savannah, Georgia, used dental records to positively identify the body as Somer's, said Clay County Sheriff's Office spokeswoman Mary Justino.

Authorities now are turning their full attention to finding the child's killer.

Beseler held up a map dotted with markers as he said on CNN's "American Morning" that detectives have questioned more than 70 registered sex offenders or predators living within five square miles of Somer's home.

The landfill where her body was found is near Folkston, Georgia, 55 miles north of where the girl became separated from her schoolmates on her way home from school in Orange Park on Monday.

The sheriff told the girl's mother Wednesday night that her daughter had been identified. Thompson, he said, was devastated.

"It was the hardest phone call I've ever had to make in my life, and I hope I never have to make another one like that," Beseler said at a Thursday morning news conference. Watch sheriff announce that girl's body identified »

Authorities will work to pinpoint where the garbage load that contained the girl's body came from, Beseler said. The garbage that was brought into that part of the landfill was collected in the Orange Park area, he had said Wednesday, noting that authorities routinely search garbage when a missing person case has been initiated.

The investigation has now turned into a murder probe.

"There is a child killer on the loose," the sheriff said. "I fear for our community until we bring this person in."

Rewards totaling $30,000 have been offered for information leading to the arrest and conviction of whoever is responsible.

CNN's Maria P. White contributed to this report.

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