Knotts told The Associated Press on Wednesday that Hill, who is now 20, lived with them for about six months in his late teens. Though there is no blood or legal connection between them, Knotts said she served as a mother-like figure to Hill.
"He was part of our family," Knotts said. Her family was aware that "he had a mental disorder" before he moved in.
Hill told her that his birth mother was dead and that he didn't know his father. He also has brothers.
Police gave more details on Wednesday about the previous day's ordeal and what led up to it. Before going to the school, investigators say that Hill took a photo of himself with an assault rifle and packed up nearly 500 rounds of ammunition enough to shoot more than half of the school's students.
Police said Hill got the gun from an acquaintance, but it's not clear if he stole it or had permission to take it.
Although no one was injured, the suspect exchanged gunfire with police who surrounded Ronald E. McNair Discovery Learning Academy in Decatur, located a few miles east of Atlanta. The school has 870 students in pre-kindergarten through fifth grade.
"We have to make a reasonable assumption he was there to do harm to someone," said DeKalb County Police Chief Cedric L. Alexander.
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 parents at a nearby Wal-Mart.
Hill held one or 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.
Hill is charged with aggravated assault on a police officer, terroristic threats and possession of a firearm by a convicted felon. Police declined to discuss what he told them when questioned.
DeKalb County Police Detective Ray Davis said Hill's motive was unclear. Hill had an address listed in court records about three miles from the school in Decatur, but no clear ties to the school.
School bookkeeper Antoinette Tuff said she was one of the employees held hostage. Tuff told WSB-TV in Atlanta that she tried to keep Hill talking to prevent him from walking into the hallway or through the school building.
"He had a look on him that he was willing to kill matter of fact, he said it. He said that he didn't have any reason to live and that he knew he was going to die today," Tuff said.
Law enforcement officers on Wednesday praised Tuff for helping to avert a tragedy.
"She was a real ally. She was a real hero in all of this. She just did a stellar job. She was cool, she was calm, very collected in all of this, maintained her wherewithal," Alexander, the police chief, said.
Tuff relayed messages from Hill to DeKalb County emergency dispatchers before convincing him to surrender. She told the dispatcher that Hill said he wasn't there to hurt the children but wanted to talk to an unarmed officer.
"He said, 'Call the probation office in DeKalb County and let them know what's going on,'" Tuff said on a 911 recording that was released Wednesday. "He said he should have just went to the mental hospital instead of doing this, because he's not on his medication."
Tuff began telling Hill of her own struggles, including raising a disabled child and losing her husband.
Tuff reassured him by saying he didn't hurt anyone, hadn't harmed her and could still surrender peacefully.
"We're not gonna hate you baby. It's a good thing that you're giving up," Tuff said after having Hill put his weapons and ammunition on the counter. Tuff told Hill that she loved him and would pray for him.
Before he surrendered, Tuff took to the school's public address system saying that Hill was sorry for what he'd done and didn't want to hurt anyone although the lockdown remained in effect.
Hill was arrested in mid-March for making terroristic threats in Henry County, DeKalb and Henry County sheriff's officials have said. He was sentenced to probation.
Tuesday's 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.
The students attended class Wednesday at nearby McNair High School, which will be used for the time being.
Knotts was shocked when she realized Hill had been taken into custody.
"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.
She kept in touch after he moved out and said he'd recently been living with another couple who belonged to the church. Knotts last saw Hill about a month ago and he seemed fine.
Knotts said Hill called her sister Tuesday afternoon before the shooting to thank the family for all they'd done for him and said he had a rifle. He didn't say what he was planning to do.
Knotts said she thinks Hill's actions were a plea for help.
"Unfortunately," she said, "he didn't know a better way to get it."