Atlanta (CNN) -- A man accused of storming into an Atlanta-area elementary school this week armed with an AK-47-type assault rifle and about 500 rounds of ammunition was arrested months ago for threatening to kill his brother, a district attorney said Wednesday.
Michael Brandon Hill barricaded himself in the front office of the Ronald E. McNair Discovery Learning Academy in Decatur on Tuesday and fired one shot inside the school and several more at police officers outside before he eventually surrendered, DeKalb County police said.
No one was injured in the ordeal.
The incident isn't the suspect's only run-in with the law. Hill, 20, was arrested in March on a charge of "terroristic threats and acts." Hill's brother reported to Henry County officials that Michael Hill had threatened on Facebook in December that he would kill him.
After pleading guilty to that charge, Hill was sentenced in July to serve three years of probation and attend anger management classes as part of a plea agreement, according to Henry County court records.
It's not clear whether Hill started the classes. Henry County District Attorney James Wright said Wednesday there is no indication he has completed them.
Ray Davis, the DeKalb County police department's lead detective on the case, said Hill has a "slight criminal history" in his county as well, without getting into specifics.
The suspect told investigators he had been "on medication and that he stopped taking it," according to Davis. Antoinette Tuff, a front office worker at the school who told media how she calmed the gunman, said on ABC News that Hill told her he "hadn't taken his medication and that he was going to die anyway and that he was OK with dying."
Authorities began trying Wednesday to talk to family, friends and acquaintances of Hill, hoping to learn more about him and what might have spurred him to do what he did.
That includes tracking down more on the gun itself, especially because it fell into the hands of someone on probation.
Hill did not own the gun, Davis said, but took it from the house of an acquaintance who police are trying to talk to, in addition to the firearm's original owner.
Charges could be finalized as soon as Thursday. Authorities are still working out exactly what charges, and how many of them, Hill will face.
A police spokesman has said the counts will include aggravated assault on a police officer, making terroristic threats and possession of a firearm by a convicted felon. Davis added that, in addition to various weapons charges, Hill is expected to be charged with a count of false imprisonment.
Hill is waiving his initial court appearance, said Claudia Saari, the circuit public defender for the Stone Mountain Judicial Circuit. She said members of her office's mental health division are handling his case, and declined to make any further comment.
CNN's Tristan Smith and Catherine E. Shoichet contributed to this report.