Atlanta (CNN) -- By any standard, the killings are grisly.
With the discovery of the body of an 87-year-old Georgia woman whose disappearance came to light after her husband's decapitated body was found in their waterfront home, authorities not only want to know who did it, but also why.
Shirley Dermond's body was found in Lake Oconee, south of Athens, Putnam County Sheriff Howard Sills told reporters Friday.
"We now, unfortunately, know that Shirley Dermond was murdered," Sills said.
Sills declined to discuss the condition of the body other than to say, "the head was not removed or anything like that."
Authorities were alerted Friday by two fishermen who spotted what they believed to be a body in the lake, some five or six miles from the couple's home.
The discovery adds an element to the investigation -- the possibility that a boat was a mode of transportation in the killing, Sills said.
"This jumps us about two jumps up on the checkerboard, shall we say," he said.
Dermond's family is in shock with the news of the discovery of her body, said the Rev. David Key of Lake Oconee Community Church.
"We haven't even had a funeral for Russell because their kids wanted to wait. Now that we know about Shirley, we will start planning the funeral services for both."
From the beginning, investigators treated Dermond's disappearance as an abduction. Her purse, cell phone and car were all at the couple's million-dollar waterfront home on Lake Oconee where her husband's headless body was found.
Investigators believe she was taken from her home after her husband, 88-year-old Russell Dermond, was decapitated sometime between May 2 and May 4.
Investigators continue to search for his head, and they plan to look in the same area of the lake where Dermond's body was found, Sill said.
"Obviously, the head of Mr. Dermond will be quite difficult to locate."
Russell Dermond's body was discovered in the couple's garage after friends, who hadn't heard from the couple in days, went to their home.
The FBI put up more than 100 billboards in its search for the missing woman, and it offered a reward of up to $20,000 for information to her location or the arrest of the person responsible for her disappearance.
"The mindset of the individual that did this was a homicidal individual that doesn't deserve to breathe the air on this Earth," Sills said.
In the days after Dermond's disappearance, authorities searched Lake Oconee in the vicinity of the couple's home -- turning up only a lawn chair and a Christmas tree -- and sent cadaver dogs into the nearby woods, to no avail. Authorities have also spoken to neighbors, family and friends.
So far, the investigation has turned up little to shed light on the crimes. Investigators aren't aware of any enemies the couple had made, or any reason someone would target them, Sills said.
CNN's Eliott C. McLaughlin and Catherine E. Shoichet contributed to this report