Michael Brandon Hill packing nearly 500 rounds at Georgia elementary school

Updated at 7:01 p.m. ET

LITHONIA, Ga. The suspect in Tuesday's Atlanta-area school shooting took a photo of himself with an AK 47-style rifle and packed up nearly 500 rounds of ammunition — enough to shoot more than half the school's students — the police said Wednesday.

Police said Michael Brandon Hill got the gun from an acquaintance, but it's not clear if he stole it or had permission to take it.

No one was injured Tuesday, but the suspect exchanged gunfire with police who surrounded Ronald E. McNair Discovery Learning Academy in Decatur. The school's 870 students in pre-kindergarten through fifth grade were evacuated.

"We have to make a reasonable assumption he was there to do harm to someone," said DeKalb County Police Chief Cedric L. Alexander.

Investigators said Hill may not have gone into the school with the idea of killing students, CBS News correspondent Mark Strassmann reports from Decatur, Ga. They said that Hill had another motive, but what that was, police won't say.

Also Wednesday, CBS News obtained a police report showing 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."

The Henry County police report 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."

Hill's brother told CBS News the suspect is bipolar and schizophrenic and has tried to kill himself several times, Strassmann reports. In June 2009, he admitted to setting his parents' house on fire.

In a 911 tape that was released Wednesday, emergency dispatchers were told during the standoff that Hill was off his medication and that he thought he should have gone to a mental hospital instead of a school.

McNair school bookkeeper Antoinette Tuff spoke for the 20-year-old Hill. She told dispatchers that Hill said he had nothing to live for and wanted police to call the DeKalb County probation office.

Hill can be heard on the call giving orders to police through his hostage.

"Tell them stand ..," Hill told Tuff, before she told the emergency dispatcher: "Tell them to stand down now. 'Tell them to stand down now,' he said."

The gunman demanded that police pull back.

"He doesn't want the kids," said Tuff. "He wants the police. So back off, and, um, and what else, sir? He said he don't care if he die. He don't have nothin' to live for. And he said he's not mentally stable."

Meanwhile, a woman whose family once took Hill in said Wednesday that he was mentally ill but never violent in the past.

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 said she served as a mother figure for Hill in after he started coming to the small church where she and her husband are pastors.

Knotts said Hill called her sister Tuesday afternoon 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 in an interview at her home in Lithonia.

Knotts said she thinks of herself as the 20-year-old Hill's adoptive mother. Hill told her that his birth mother was dead and that he didn't know his father. He also has a brother.

Hill held two staff members in the front office captive for a time, the police chief said, making one of them call a local TV station. At some point, he fired into the floor of the school office. As officers swarmed the campus outside, he shot at them at least a half a dozen times with an assault rifle from inside the school and they returned fire, police said. Police came into the school office, and Hill surrendered.

"I just want you to know that I love you, though, OK?" Tuff was heard telling Hill on the 911 recording. "And I'm proud of you. That's a good thing that you just given up and don't worry about it. We all go through something in life."

Hill is charged with aggravated assault on a police officer, terroristic threats and possession of a firearm by a convicted felon. Police questioned him for hours at headquarters, but declined Wednesday to discuss what he said.

DeKalb County Police Detective Ray Davis said a court date could take place in the coming days.

Davis said Hill's motive was unclear but he'd had contact with the school office before. Hill had an address listed in court records about three miles from the school in Decatur, but no clear ties to the school.

The ordeal terrified parents.

Rufus Morrow was at work when he got a phone call with news that shots had been fired at the school his daughter attends.

He drove "about 90 mph" to the school. The police chief says Hill, armed with an assault rifle and other weapons, was able to slip into the school where visitors must be buzzed in by staff.

Morrow said he almost cried as he told his supervisor why he needed to leave.

"Just the mere thought of what happened at that other elementary school happening here, it was just devastating to my soul," he said, referring to the Sandy Hook Elementary School massacre in Connecticut in December that left 26 people dead, 20 of them children.

Dramatic television footage showed lines of young students racing out of the building with police and teachers escorting them to safety. They sat outside in a field for a time until school buses came to take them to their waiting parents and other relatives at a nearby Wal-Mart.

Morrow was one of those parents and held his 10-year-old daughter close to him during an interview after the two were reunited.

"My stomach was in my throat for the whole time until I saw her face on the bus," he said.

His daughter, a fifth-grader named Dyamond, told The Associated Press that a voice came over the intercom saying school was under lockdown and instructed students to get under tables. She said her teacher told the class to sing and pray.

"There were a lot of girls crying, I was feeling scared but I didn't cry. I was just nervous," she said.

Complicating the rescue, bomb-sniffing dogs alerted officers to something in the trunk of a car in the school parking lot. They thought the suspect may have brought explosives, though the car turned out not to be his and had no explosives. Authorities said Wednesday that the dogs smelled a legal product that can be used to make explosives.

Students at the school arrived Wednesday morning at nearby McNair High School, where they would attend classes for the time being. The high school's marquee said "Welcome McNair Elementary School Our Prayers Are With You." The academy is named after McNair, an astronaut who died when the space shuttle Challenger exploded on Jan. 28, 1986, according to the school's website.