(CNN) -- The FBI is offering a reward in its search for the wife of an 88-year-old Georgia man who was found decapitated.
It will pay "up to $20,000 for information leading to the location of Shirley Wilcox Dermond and/or the arrest of the individual(s) responsible for her disappearance," Special Agent Stephen Emmett of the FBI's Atlanta office said Wednesday.
Investigators are treating Dermond's disappearance as an abduction, Putnam County Sheriff Howard Sills has said. Investigators believe Russell Dermond of Eatonton, Georgia, was killed between May 2 and May 4, and they're still searching for his severed head, Sills told reporters last week.
With no breakthrough in the case, authorities are reaching out to commuters. The FBI commissioned at least 100 billboards throughout the state in its search for 87-year-old Shirley Dermond and her husband's killer.
The digital billboards say, "Missing and in danger," and include a photo of the gray-haired, blue-eyed Dermond, along with her height, 5 feet 2 inches tall, and weight, 148 pounds.
Anyone with information is encouraged to call the FBI's Atlanta field office at 404-679-9000 or the Putnam County Sheriff's Office at 706-485-8557.
"They're basically running in all corners of the state," said the Outdoor Advertising Association of Georgia's executive director, Conner Poe. "They'll run once a minute, every minute, 24/7."
The billboards can be seen as far south as Valdosta, as far east as Brunswick, and as far north as Chattanooga, Tennessee, with 50 in metro Atlanta alone, Poe said.
There is no indication that Shirley Demond is a suspect, and there's "plenty" to indicate she was taken, Sills said. Her pocketbook, cell phone and vehicle were all at the million-dollar waterfront home.
"I don't think it's a random incident. I think for whatever reason these people were singled out for this," Sills said. "They live in the most exclusive neighborhood, or one of the most exclusive neighborhoods, in this county. ... They're on a cul-de-sac in a gated, multimillion-dollar resort community that we have no crime in."
The Dermonds' friends hadn't heard from them in days and went to their home in the lakeside Great Waters community, where they found Russell Dermond's headless body in the garage.
Authorities have searched Lake Oconee in the vicinity of the Dermonds' home -- turning up only a lawn chair and a Christmas tree -- and sent cadaver dogs into the nearby woods, to no avail, Sills said.
Residents described Reynolds Plantation -- a tony resort complex about 75 miles east of Atlanta, which also boasts a Jack Nicklaus signature golf course and a Ritz-Carlton Lodge -- as a safe enclave where people were comfortable leaving their doors unlocked.
The 3,300-square-foot home where the Dermonds had lived since 1994 is valued at more than $1 million.
The Rev. David Key of Lake Oconee Community Church last week said he has known the couple for about eight years and counts them among his church's 350 attendees. He said he's "baffled" as to why anyone would commit such crimes against them.
He described the couple of 68 years, who had three adult children, as grounded, "beloved in the community" and "sweet as can be."
CNN's Vivian Kuo, Devon M. Sayers, AnneClaire Stapleton and John Murgatroyd contributed to this report.