DeKalb County Police Detective Ray Davis says police believe that Hill got the AK 47-style rifle he carried from an acquaintance. But he said at a news conference on Wednesday that it's not clear if he stole the gun or borrowed it.
Hill was arrested earlier this year for allegedly threatening on Facebook to shoot his older brother in the head "and not think twice about it," according to a police report obtained by CBS News.
The 20-year-old man faces multiple charges after police say he walked into Ronald E. McNair Discovery Learning Academy Tuesday armed and held one or two staff members in the front office captive. On Tuesday, as officers swarmed the elementary school campus, DeKalb County Police Chief Cedric L. Alexander said he shot at them at least a half a dozen times with an assault rifle from inside the school and they returned fire. Michael Hill then surrendered. No one was injured.
A Henry County police report obtained by CBS News Wednesday shows that Hill's older brother, Timothy Hill, told police last New Year's Eve that threats Michael made a day earlier made Timothy "fear for his life."
"Mr. Hill advised his brother stated on Facebook that he would shoot him in the head and not think twice about it," a police officer wrote in the report.
Timothy Hill told police that he knew his younger brother had "mental issues" and was under a doctor's care but that he didn't know where Michael was living at the time.
More than two months later, in March, Michael Hill turned himself in on the outstanding warrant for making "terroristic threats."
A woman who took care of the suspect in the past said he was mentally ill but never violent.
Natasha Knotts told The Associated Press that Hill lived with her and her husband for a time when he was in his late teens. She says they took him in after he started coming to the church where they serve as pastors.
She says Hill called her sister Tuesday before the shooting and said he had a rifle but didn't say what he was planning to do. She said she believes that Hill acted out as a plea for help.
"This is something that's totally out of his character. This is not him. This is not the Mike that I know. For anyone that knew Mike, this was a total devastation," she said.
Hill told her that his birth mother was dead and that he didn't know his father.