ATLANTA (REUTERS) - A 20-year-old man who opened fire with an AK-47 in an Atlanta-area elementary school has a history of mental illness and once threatened in a Facebook message to kill his brother, according to the suspect's lawyer and family.

A school clerk convinced Michael Brandon Hill to lay down his gun and turn himself in after he walked into the school carrying 500 rounds of ammunition and exchanged gunfire with police. No students were injured in the incident.

"Mr Hill is being represented by members of our Mental Health Division," said Ms Claudia Saari, a public defender representing Hill.

Mr Timothy Hill, the brother of the alleged gunman, told police he received threatening messages on Facebook from his brother, including one that stated he would "shoot him in the head and not think twice about it," a December 2012 police report said.

Mr Timothy Hill told police "he was in fear for his life" and wanted to pursue charges against his brother, the report said.

He said his brother had mental issues and was under a doctor's care, according to the report.

In July, Michael Hill was sentenced to three years probation and ordered to attend anger management classes for the threat made against his brother, according to Henry County Court records.

On Thursday, students at the Ronald E. McNair Discovery Learning Academy returned to the school for the first time after the shooting, said Mr Quinn Hudson, spokesman for the DeKalb County school system.

Crisis counselors were on hand to talk with students, he said.

Students who talked to counsellors were not afraid but wanted "reassurance that everything is OK," Mr Hudson said.

The incident follows the December killing of 20 children and six adults by Adam Lanza, a young man with a history of mental illness, who used an assault rifle in the massacre at an elementary school in Connecticut.