The 21-year-old stepson apparently shot himself Monday night after shooting the 52-year-old deputy as he slept, the deputy's 36-year-old wife and the couple's 12-year-old daughter, authorities said Tuesday.
Each victim had a single gunshot wound, and no note was found, authorities said.
Deputy Sam Driskell, a detective in charge of the sex offenders registry, had earlier been in the military for 20 years, and he met his wife, Muachin, while he was in the Dominican Republic, authorities said.
The stepson was identified as Felix Almonte, authorities said.
"There's no rhyme or reason for it. All of my guys are heartbroken," Paulding County Sheriff Gary Gulledge told reporters.
The sheriff knew the slain deputy "all my life," he said.
"He's here early and leaves late," the sheriff said of Driskell. "He's always the first one on the job."
The couple had a daughter, Carolyn, together, authorities said. She was the 12-year-old also found killed.
"What we believe was they had some issue with the stepson," the sheriff said of the family.
There were no signs of domestic violence, the sheriff added.
Driskell, a corporal, worked for the sheriff's office for more than 10 years and had been recognized by the Georgia Bureau of Investigation for his work fighting sex crimes against children, authorities said.
Driskell didn't show up for court Tuesday, which was uncharacteristic of him, and his body and the others were discovered when deputies went to check on him, authorities said.
Neighbors expressed deep concern Tuesday.
"My grandpa called to let us know stay in the house, lock the doors," Haley Smallwood told CNN affiliate WSB.
"They told us everything they knew, that they were dead and they were found in the house.
"They told us to be careful," she added.