She identified the victims as Gwinnett County Sheriff's Deputy Joy Deleston, 39, and her two daughters, Micaiah, 11, and Jelani, 4. The motive was still unclear.
Deleston brought a paternity suit in 2004, claiming that the 32-year-old rap star Juvenile, whose real name is Terius Gray, was Jelani's father, Gwinnett County court records show.
Attorney Randy Kessler, who represented Juvenile in the proceedings, said the case was resolved peacefully by consent order in 2006. Both parties agreed Juvenile was the father.
Terrell was taken to jail in neighboring DeKalb County to avoid any potential conflict because Deleston worked for Gwinnett County, Spellman said. He is being held without bail.
Because of his age, prosecutors cannot seek the death penalty against Terrell. A preliminary hearing will be set next week in Gwinnett County Superior Court, said prosecutor Danny Porter.
Defense attorney Lyle Porter did not return a call Friday, and a number was not immediately available for Juvenile or a spokesperson.
Authorities on Friday found the gun used, going by information provided by Terrell. Spellman said she could not confirm whether Deleston's service weapon was used.
Deleston, a seven-year member of the department, had been assigned to the sex offender unit, officials said.
Police had gone to the subdivision because a caller reported shots fired, Officer David Schiralli said earlier. They found a bullet hole in a house, then checked next door because they knew a deputy lived there.
"There was no answer, but lights were on," Schiralli said. "They opened the door a little bit to announce themselves," then discovered the bodies, he said.
The 17-year-old son arrived at the house about 35 minutes later, he said. Spellman said he was arrested after talking to investigators but didn't elaborate.
Neighbors said the family had moved in less than a year ago. They often saw Deleston in her squad car coming home from work and said having a deputy around was reassuring.
Jeff Beaird, who lives across the street, said he often saw Terrell in the driveway shooting hoops with his friends or washing his mother's patrol car but doesn't remember anything out of the ordinary.
"It's so shocking because you don't expect something like this to happen," Beaird said.
Michael Hunt said he was taking a nap before starting work on the night shift Thursday when he heard the news about his neighbors. Hunt recalled that his wife had seen Terrell hours earlier walking down the street carrying a basketball.
"For him to be scared of our dog and then to kill his whole family ... it just doesn't add up," Hunt said.
Juvenile, a rapper based in New Orleans, has been arrested several times on charges ranging from failure to pay child support to resisting an officer in a scuffle at a Florida mall in 1999. Most of the charges have been dismissed.