The body of Christopher Barrios was found Thursday afternoon inside the bag, three miles from where he was last seen at his Glynn County trailer park.
"It was obvious," said Cpl. Jesse Cook of the Department of Natural Resources. "But if you weren't looking for it, you probably would've thought it was just a trash bag."
Glynn County Police Chief Matt Doering said he escorted the boy's father, Mike Barrios, to the scene to see the body at the father's request.
Volunteers, wearing T-shirts printed with the boy's photo, sobbed as they heard the news.
"You suspected all along in your heart, it's just not the outcome you want," said volunteer searcher Mari Charnock, who dabbed at tears after the boy was found dead. "At least we know, though. At least it's over."
Chief Doering said he expects to charge all four suspects arrested earlier in the week with Christopher's murder. The boy had obviously been slain, he said, but he would not elaborate.
The body was taken to the Georgia Bureau of Investigation crime lab in Savannah, spokesman John Bankhead said Friday.
Christopher lived with his father in a trailer park of about 50 homes. He often stayed with his grandmother, whose trailer is 200 feet from his father's home.
Earlier this week, police arrested 32-year-old George David Edenfield, who lived with his parents across the street from the boy's grandmother, after Mr. Edenfield told investigators he played a role in the abduction.
Mr. Edenfield was charged with violating his probation from a 1997 child molestation conviction, which prohibited him from contact with children younger than 18.
His parents, David and Peggy Edenfield, also were arrested after giving police conflicting statements on where they might find the boy. They were charged with lying to police and obstruction for initially telling police they knew nothing about the disappearance. The same charge was brought against a family friend, Donald Dale. Mr. Dale and David Edenfield told investigators they had buried the boy's body.