ORANGE PARK, Fla. -- Authorities in North Florida say they will search a Georgia landfill throughout the night and into the morning for evidence, after an unidentified child's body was found.

Clay County sheriff's investigators hope to identify the child's body on Thursday.

The discovery of the body worsened the community's fears that it could be 7-year-old Somer Thompson, who disappeared on Monday.

Thompson's mother tearfully thanked hundreds of people who showed up at a vigil for the missing girl Wednesday night.

"I don't know how I can ever repay any of you for helping me, for looking for my baby," Diena Thompson told the crowd.

The body was found by Clay County detectives who followed garbage trucks from the girl's neighborhood in Orange Park, Florida to Folkston, Georgia, just north of the Florida state line.

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RAW VIDEO: Sheriff Announces Body Found
EMOTIONAL NEWS CONFERENCE: See Images | Raw Video
IMAGES: Photos Of Somer Thompson
GO TO: Amber Alert Website

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Beseler said investigators searched through 100 tons of garbage before finding the partially covered body. He did not give any other details about the discovery.

The Georgia Bureau of Investigation is helping with the investigation and planned to conduct an autopsy on the body Thursday in its Savannah office, spokesman John Bankhead said.

"We all need to say a prayer for Somer's family," Florida Gov. Charlie Crist said at a press conference announcing the discovery.

Somer vanished on her mile-long walk home from school Monday 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.

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.

Investigators said before the body was discovered they suspected foul play in her disappearance.

"We want to find who did this and bring them to justice," the sheriff said.

Earlier Wednesday, the girl's parents made emotional pleas for their daughter to be returned home safely.

"Somer, you're not in trouble -- if you're scared, just come home," Diena Thompson said on NBC's "Today" show Wednesday. "And if somebody has her, please, please bring her back to us. She belongs with her family."

Father Sam Thompson, who lives in Graham, N.C., also pleaded for her safe return.

"Somer, your daddy Sam loves you unconditionally. Stay strong and don't give up the fight or the hope that we're going to be a family again. I love you," he said.