ORANGE PARK, Fla. -- Authorities have tentatively identified a body found in a Georgia landfill as that of 7-year-old Somer Thompson, a north Florida girl who disappeared on her walk home from school, the sheriff in charge of the case said Thursday.

Sheriff Rick Beseler said the tentative identification was based on clothing and on a birthmark that matched the girl's.

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SLIDESHOW: Somer Thompson Pictures

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The partially covered body of the child was found Wednesday in a Georgia landfill near the Florida state line, after investigators followed garbage trucks leaving the neighborhood where the child disappeared Monday.

Beseler said at a news conference he fears for the community until a suspect is apprehended. He wouldn't talk about what evidence police have recovered, or whether investigators believe the crime was committed by one or more people. He said police have questioned more than 70 registered sex offenders in the area, and that process was continuing.

"I fear for our community until we bring this person in. This is a heinous crime that's been committed," Beseler said. "And we're going to work as hard as we can to make this community safe."

The sheriff said he had told the girl's mother to prepare for the worst, and called her after receiving the news Wednesday night.

"Needless to say, she was absolutely devastated," he said.

"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."

An FBI forensic unit is helping process evidence from the landfill in Folkston, Ga., about 48 miles from where the girl disappeared.

The Georgia Bureau of Investigation planned an autopsy Thursday in Savannah.

Somer vanished on her mile-long walk home from school in Orange Park. She was squabbling with another child, and her sister told her to stop. The girl got upset, walked ahead of the group and wasn't seen again.

Authorities launched a countywide search involving helicopters, dogs and volunteers walking arm-to-arm through wooded areas.

Orange Park is a suburb of Jacksonville just south of Jacksonville Naval Air Station. The area where the girl disappeared is a heavily populated residential area with homes, apartment complexes and condominiums.

The girl's father, Sam Thompson, lives in Graham, N.C.