Thornton opened fire shortly after being forced to resign from Hartford Distributors on Aug. 3. Company officials said they let him go because he was caught on tape stealing beer. They say Thornton, 34, never complained of racial discrimination at work.
"I know he reported the problems because he brought home the papers that they signed," Holliday told CNN's Soledad O'Brien.
She said her son called her during the shooting spree. "He said, 'I just killed the five most racist people,' and I knew he wasn't playing because he don't talk like that," Holliday said.
Holliday told CNN she saw changes in her son in the days and weeks leading up to the shootings. "I could just kind of see slowly he was just getting frustrated and aggravated," she said. "He started not being so relaxed but I never thought that it would be possible that something like this would happen."
Thornton appeared calm during a 911 call he made to police after the killings. His mother said that was his nature. "I think that he was at his limit and the calmness is something that he's always been. I've really never seen him enraged," Holliday told CNN.