Plenty of evidence links the killings, though officials have not yet determined how the victims are connected or if they knew whoever shot them, said Cherokee County Sheriff Bill Blanton.
"Yes, we have a serial killer," he said at a news conference in this rural community 50 miles south of Charlotte, North Carolina.
So far, all investigators have to go on is a sketch of a suspect and a description of a possible getaway vehicle, though police would not say who provided that information.
The latest victims were found in their family's small furniture and appliance shop near downtown Gaffney around closing time Thursday. Stephen Tyler, 45, was killed, and his 15-year-old daughter was shot and seriously injured. Tyler's wife, his older daughter and an employee found them in Tyler Home Center, County Coroner Dennis Fowler said.
A day earlier and about 7 miles (11 kilometers) away, family members found the bodies of 83-year-old Hazel Linder and her 50-year-old daughter, Gena Linder Parker, bound and shot in Linder's home. Blanton would not say if Tyler and his daughter were also bound.
The killing spree began last Saturday about 10 miles (16 kilometers) from Tyler Home Center, where peach farmer Kline Cash, 63, was found shot in his living room. Blanton said the killer may have first spoken with Cash's wife about buying hay. She left and came home a few hours later to find her husband's body. Investigators said it appears he was robbed, but they have not determined if anything was taken in the other killings.
Cherokee County, home to about 54,000 people, had just six homicides in all of 2008, and half that the year before.
Residents have "their guard up and their gun handy," said state Sen. Harvey Peeler, who recalled the area being terrorized once before, in the 1960s, by a serial killer dubbed the Gaffney Strangler. Otherwise, Gaffney is most famous for a giant water tank shaped like a peach that can be seen from Interstate 85.
"You can cut the tension with a knife," Peeler said. "People are locking their doors, even in broad daylight."
The Fourth of July creates a busy weekend, with thousands of people expected to attend fireworks displays in several communities.
Tanya Phillips had been looking forward to a backyard barbecue at her brother's house but instead planned to stay home with her doors locked.
"I'm not taking any chances," said Phillips, 32, a mother of two who works in a day-care center. "I'll go out during the day, but not at night. I just don't feel safe."
"You want to live a normal life," Phillips said as she stood outside a grocery store. "But you just can't."
Every available police officer will work the weekend, Blanton said, acknowledging that there is "real fear in the county." He urged people to take precautions such as going out in groups and calling police if their cars break down and they are stuck on the side of the road.
"If someone breaks into my house, I'm armed and ready," said Mike Daniels, 53, a retired Army sergeant. "And I won't hesitate to shoot first and ask questions later."
The latest shootings happened less than a half-mile (800 meters) from the sheriff's office, where at least 30 investigators were already working on the case. Blanton said a profiler has suggested Tyler and his daughter might have been shot to taunt investigators, but he said his only concern is solving the case.
The sheriff reminded people they have a right to protect themselves and advised salesmen and others to avoid knocking on strangers' doors with so many on heightened alert.
"People are going to start shooting at shadows," Blanton said.