The indictments charge 15 members of an organization known as Respect the Flag with violating Georgia's Street Gang Terrorism and Prevention Act and making terroristic threats. If convicted, they could face up to 20 years in prison.
Melissa Alford, who hosted the July birthday party, where many of the attendees were black, said tensions flared when the trucks drove by the home in Douglasville, Georgia. The people holding the flags were also brandishing weapons, she said.
"They used the N-word and said they were going to kill people on my property," Alford told CNN affiliate WSB.
Members of Respect the Flag have described the day's events differently, telling the Atlanta Journal-Constitution that someone at the party threw rocks at them, triggering the altercation.
The group's leader, Levi Bush, who is among the people indicted, declined CNN's request for an interview. Other members of the group could not be reached for comment, and it was not clear whether they had legal representation.
The indictment alleges that Respect the Flag is a "criminal street gang," and that members of the group threatened "to commit a crime of violence" against people at the party, "with the purpose of terrorizing those individuals and in reckless disregard for the risks of causing such terror."
The prosecutor has said he won't discuss details of the case outside court.
"It would be inappropriate for us to get into the facts and details of the case," Douglas County District Attorney Brian Fortner told reporters this week. "I think that's better reserved for the courtroom."
The Southern Poverty Law Center, which is representing some of the people who attended the party, praised the prosecutor for pushing forward with the case.
"These cowards chose unarmed African-Americans enjoying a peaceful birthday party to vent their violent racist hatred," Morris Dees, the organization's chief trial counsel, said in a written statement. "This is reminiscent of the Ku Klux Klan -- modern-day night-riders terrorizing African-Americans in the name of Southern heritage."