(NEW YORK) -- The mother of Colt Gray, the 14-year-old suspected of opening fire at his Georgia high school, said she last heard from her son the morning of the shooting when he texted her, "I'm sorry, Mom."
"His dad had actually gotten texts from him that basically said the same thing already that morning, about 15 minutes prior," Marcee Gray told ABC News. "One said, 'I'm sorry,' and one said, 'You're not to blame for this.'"
Marcee Gray said she was "petrified" by the texts to her husband and called the school, and that's when she received the "I'm sorry, Mom," message.
"My concern for him had been building," she said.
Marcee Gray said she'd called her son's school six days earlier because "I wanted Colt to be admitted to an impatient treatment. ... Colt was on board with it."
Colt Gray is accused of killing two students and two teachers, and injuring nine others, at Apalachee High School on Wednesday.
Marcee Gray said when she first heard about the shooting, she "fell to the ground and just started screaming. ... I knew what had happened, I just knew in my gut."
Nearly one week later, she said her feelings range from "debilitating anxiety" to "guilt" to "disgust."
"I still can't believe it happened," she said.
Marcee Gray said she wants the victims' families to know "how horrible I feel."
"What happened to them and their sweet, innocent babies is just unfathomable," she said. "If I could take their place, I would. I would in a heartbeat."
Marcee Gray said if she had the chance to talk to her son now, she'd tell him, "I love him," and, "It's not your fault." She did not say who she places the blame on.
Colt Gray is charged with four counts of felony murder. More charges will be filed, prosecutors said.
The teen's father, Colin Gray, is charged with four counts of involuntary manslaughter, two counts of second-degree murder and eight counts of cruelty to children, the GBI said. He is accused of "knowingly allowing his son, Colt, to possess a weapon," GBI Director Chris Hosey said.
Investigators believe Colt Gray received the AR-style gun used in the shooting as a Christmas present from his father, according to sources.
"I knew that my husband had numerous guns, but I didn't know that one was a specific Christmas present," Marcee Gray said. "It wasn't until after the shooting that I knew it was specifically Colt's gun. ... I was shocked."
The father and son both made their first court appearances on Friday. Neither has entered a plea and both are set to return to court on Dec. 4.