The motives of Michael Brandon Hill, the suspect in Tuesday's Georgia elementary school shooting, remain a mystery as officials try to establish what provoked him.
No one was hurt when Hill, 20, entered the Ronald E. McNair Discovery Learning Academy in Decatur, Ga., with an AK-47 assault-style rifle.
Authorities say Hill shot about six rounds at law enforcement personnel who raced to the school, but surrendered peacefully when they returned fire.
It wasn't his first alleged encounter with police officers. In December, Hill "threatened to shoot" his brother, Timothy Hill, Timothy told NBC News.
The threat was in December, just two weeks after the school shooting in Newtown, Conn., which left 26 people dead, 20 of whom were children.
A grand jury charged Hill with terroristic threats for the incident. He received three years of probation and was ordered to be treated for anger management.
In addition, Timothy Hill said his brother was issued a no-contact order.
Hill is "someone who slipped through the cracks," Timothy said, describing his brother as someone who started "having problems" at age 13.
"He had his good times, and then he had his bad times," Timothy said.
"He was bipolar and suffered from ADD."
He didn't say whether that was an official diagnosis.
Hill shared a rental home in Decatur with a woman and her boyfriend and up to three other residents. The woman, who was not identified, told Today:
"I've been having a rough time. I just pray for him."
Neighbors described Hill to the Atlanta Journal-Constitution as quiet and polite, and said he and the others moved into the house about a year ago.
The residents sometimes threw small get-togethers in the wooded neighborhood home but weren't particularly loud, neighbors told the paper.
The house is within walking distance of the the Ronald E. McNair Discovery Learning Academy. Whether Hill had a connection to it is unclear.
Hill is charged with aggravated assault on a police officer, making terroristic threats and possession of a firearm by a convicted felon for the shooting.