The Census Bureau has suspended door-to-door interviews in rural Clay County, where the body was found, pending the outcome of the investigation. An autopsy report is pending.
Investigators have said little about the case. FBI spokesman David Beyer said the bureau is assisting state police and declined to confirm or discuss any details about the crime scene.
"Our job is to determine if there was foul play involved - and that's part of the investigation - and if there was foul play involved, whether that is related to his employment as a Census worker," said Beyer.
Attacking a federal worker during or because of his federal job is a federal crime.
Sparkman's mother, Henrie Sparkman of Inverness, Fla., told The Associated Press her son was an Eagle scout who moved to the area to be a local director for the Boy Scouts of America. She said he later became a substitute teacher in Laurel County and supplemented that income as a Census worker.
She said investigators have given her few details about her son's death - they told her the body was decomposed - and haven't yet released his body for burial. "I was told it would be better for him to be cremated," she said.
Henrie Sparkman said her son's death is a mystery to her. "I have my own ideas, but I can't say them out loud. Not at this point," she said. "Right now, what I'm doing, I'm just waiting on the FBI to come to some conclusion."
Gilbert Acciardo, a retired Kentucky state trooper who directs an after-school program at the elementary school where Sparkman was a frequent substitute teacher, said he had warned Sparkman to be careful when he did his Census work.
"I told him on more than one occasion, based on my years in the state police, 'Mr. Sparkman, when you go into those counties, be careful because people are going to perceive you different than they do elsewhere,"' Acciardo said.
"Even though he was with the Census Bureau, sometimes people can view someone with any government agency as 'the government.' I just was afraid that he might meet the wrong character along the way up there," Acciardo said.
Acciardo said he became suspicious when Sparkman didn't show up for work at the after-school program for two days and went to police. Authorities immediately initiated an investigation, he said.
"He was such an innocent person," Acciardo said. "I hate to say that he was naive, but he saw the world as all good, and there's a lot of bad in the world."
Lucindia Scurry-Johnson, assistant director of the Census Bureau's southern office in Charlotte, N.C., said law enforcement officers have told the agency the matter is "an apparent homicide" but nothing else.
Census employees were told Sparkman's truck was found nearby, and a computer he was using for work was found inside it, she said. He worked part-time for the Census, usually conducting interviews once or twice a month.
Sparkman has worked for the Census since 2003, spanning five counties in the surrounding area. Much of his recent work had been in Clay County, officials said.
Door-to-door operations have been suspended in Clay County pending a resolution of the investigation, Scurry-Johnson said.
The Census Bureau has yet to begin door-to-door canvassing for the 2010 head count, but it has thousands of field workers doing smaller surveys on various demographic topics on behalf of federal agencies. Next year, the Census Bureau will dispatch up to 1.2 million temporary employees to locate hard-to-find residents.
The Census Bureau is overseen by the Commerce Department. "We are deeply saddened by the loss of our co-worker," Commerce Secretary Gary Locke said in a statement. "Our thoughts and prayers are with William Sparkman's son, other family and friends."
Locke called him "a shining example of the hardworking men and women employed by the Census Bureau."
Appalachia scholar Roy Silver, a New York City native now living in Harlan County, Ky., said he doesn't sense an outpouring of anti-government sentiment in the region as has been exhibited in town hall meetings in other parts of the country.
"I don't think distrust of government is any more or less here than anywhere else in the country," said Silver, a sociology professor at Southeast Community College.
The most deadly attack on federal workers came in 1995 when the federal building in Oklahoma City was devastated by a truck bomb, killing 168 and injuring more than 680. Timothy McVeigh, who was executed for the bombing, carried literature by modern, ultra-right-wing anti-government authors.
A private group called PEER, Public Employees for Environmental Responsibility, tracks violence against employees who enforce environmental regulations, but the group's executive director, Jeff Ruch, said it's hard to know about all of the cases because some agencies don't share data on instances of violence against employees.
From 1996 to 2006, according to the group's most recent data, violent incidents against federal Bureau of Land Management and Forest Service workers soared from 55 to 290.
Ruch said that after the 1995 bombing of the federal building in Oklahoma City, "we kept getting reports from employees that attacks and intimidation against federal employees had not diminished, and that's why we've been tracking them."
"Even as illustrated in town hall meetings today, there is a distinct hostility in a large segment of the population toward people who work for their government," Ruch said.
Barrett reported from Washington. Associated Press writers Roger Alford in Frankfort, Ky., Hope Yen in Washington and Dylan T. Lovan in Louisville contributed to this report.