A man who said he was among those who found the body told tells the Associated Press that Sparkman was naked, bound at the hands and feet with duct tape and gagged - details that have not yet been confirmed by authorities.
Jerry Weaver of Ohio told the Associated Press he was visiting a cemetery in rural Kentucky with family members on Sept. 12 when he, his wife and daughter saw the body.
"The only thing he had on was a pair of socks," Weaver said. "And they had duct-taped his hands, his wrists. He had duct tape over his eyes, and they gagged him with a red rag or something.
"He was murdered," Weaver said. "There's no doubt."
Weaver said the body was about 50 yards from a 2003 Chevrolet S-10 pickup truck.
Two people briefed on the investigation said various details of Weaver's account matched the details of the crime scene, though both people said they were not informed who found the body. The two spoke on condition of anonymity because they were not authorized to discuss the case.
Authorities have said a preliminary cause of death was asphyxiation, pending a full medical examination. According to a Kentucky State Police statement, the body was hanging from a tree with a rope around the neck, yet it was in contact with the ground.
"And they even had duct tape around his neck," Weaver said. "And they had like his identification tag on his neck. They had it duct-taped to the side of his neck, on the right side, almost on his right shoulder."
Both of the people briefed on the investigation confirmed that Sparkman's Census Bureau ID was found taped to his head and shoulder area. Weaver said he couldn't tell if the tag was a Census ID because he didn't get close enough to read it. He could see writing on Sparkman's chest, but could not read that it said "fed."
(Left: A 2008 photo of Bill Sparkman speaking to a 7th grade class.)
A retired state trooper who worked with Sparkman at Johnson Elementary School, some 30 miles from where the body was found, said he warned Sparkman that some people in the rural area are hostile.
"I said, 'Bill you be careful when you go over to eastern Kentucky to do your census work. Some of the people over there may not understand that you're just collecting statistics,'" friend Gilbert Acciardo told CBS News.
Clay County Sheriff Kevin Johnson declined to comment on the investigation because the department is only playing a supporting role but said patrols have increased in the Daniel Boone National Forest since the body was found.
State Trooper Don Trosper said it was clear this wasn't a natural death but said all other possibilities were being considered.
"We are not able to rule out many scenarios at this time, and that's what makes this a difficult case," he said.
According to the sheriff, there's only been one murder in Clay County in three years, reports CBS News correspondent Dave Browde.
Although anti-government sentiment was one possibility in the death, some in law enforcement also cited the prevalence of drug activity in the area - including meth labs and marijuana fields - although they had no reason to believe there was a link to Sparkman's death.
After Sparkman's body was found, the Census Bureau suspended door-to-door interviews in Clay County until the investigation is complete.