Two people survived the attack, with brutal injuries.
Police have not detailed what they found at the mobile home nestled among centuries-old, moss-draped oak trees in coastal southeast Georgia. Glynn County Police Chief Matt Doering has only said "it's not a scene that I would want anybody to see."
Guy Heinze Jr., 22, was arrested late Saturday and charged with illegal possession of prescription drugs and marijuana, tampering with evidence and making false statements to police, Doering said. Doering did not know whether he had an attorney.
"He was a family member who came home and discovered (the victims), at least that's what he told us," Doering told reporters. "He was the one who called 911."
Asked if police believe Heinze was involved in the slayings, Doering said: "I'm not going to characterize him as a suspect."
The chief also said police didn't know whether more than one person was involved.
Police have released few details about the mass slaying in this coastal Georgia county. Seven people were found dead along with two critically injured survivors Saturday morning at the trailer park on the grounds of a historic plantation.
Autopsies were being conducted Sunday by the Georgia Bureau of Investigation. The two surviving victims remained in critical condition at a hospital in Savannah, 60 miles north of the crime scene, Doering said.
Police have not released the victims' names or said how they died, but Doering said neither the dead nor the injured committed the killings.
"We're comfortable that none of those nine were involved with this assault," he said.
Investigators were talking to neighbors about whether they saw or heard anything unusual at the home, where an old boat sat in the front yard. Police had not been able to speak with the survivors, who may be the only witnesses.
All seven bodies were tentatively identified and Doering said families of the victims had been notified, but he would not release names or ages before receiving the autopsy results. Earlier, he said some of the victims were in their teens.
The 1,100-acre mobile home park is all that remains of a Crown grant made in 1763 to Henry Laurens, who later succeeded John Hancock as president of the Continental Congress in 1777.
Laurens obtained control of the South Altamaha river lands and named it New Hope Plantation, according to the plantation's Web site.
Lisa Vizcaino, who has lived at New Hope for three years, said the management works hard to keep troublemakers out of the mobile home park and that it tends to be quiet.
"New Hope isn't rundown or trashy at all," Vizcaino said Saturday. "It's the kind of place where you can actually leave your keys in the car and not worry about anything."
Vizcaino said she didn't know the victims and heard nothing unusual when she woke up at 7 a.m. Saturday morning. After word of the slayings spread, she said, the park was quieter than usual.
"Everybody had pretty much stayed in their houses," Vizcaino said. "Normally you would see kids outside, but everybody's been pretty much on lockdown."