Atlanta (CNN) -- The mother of the teenager suspected of killing four people during last week's shooting at a Georgia high school has apologized to the victims' families in an open letter while insisting her son "is not a monster."

"To the parents and families of those affected by the tragic events at Apalachee High School, I want to say that I am so sorry from the bottom of my heart," Marcee Gray, the mother of 14-year-old Colt Gray, wrote in the letter, which she provided first to CNN.

The letter comes one week after the shooting, which authorities allege was carried out by Colt, leaving two teachers and two students dead at Apalachee High School in Winder, Georgia. Seven others were wounded by being shot, and two others suffered other injuries, authorities said.

Colt Gray is charged with four counts of felony murder and will be tried as an adult.

"If I could take the place of Mason and Christian, I would without a second thought," Marcee Gray wrote, referring to the two 14-year-olds killed in the mass shooting. She later added her "heart breaks for the 2 teachers who gave their lives while in the service of teaching and protecting our children," acknowledging Richard Aspinwall, 39, and Cristina Irimie, 53, who were also killed.

"We are all in a living nightmare right now, and I will personally never forgive myself for what has happened," she wrote. "My son Colt is not a monster. He is my oldest baby. He is quiet, thoughtful, caring, funny, and extremely intelligent. Please pray for him and the rest of our family, as I am praying for all of you every moment of every day."

CNN is reaching out to victims' families for comment on the letter.

Over the past week, revelations about a call between Marcee Gray and the school prior to the attack have raised questions about efforts by school officials and law enforcement to prevent the shooting.

Marcee Gray received a text the morning of the shooting from her son that read, "I'm sorry, mom," her father previously told CNN. It was concerning enough that Marcee Gray called the school to warn of an "extreme emergency" at 9:50 a.m., about 30 minutes before police responded to the school for a shooting, according to call logs and a text exchange between Marcee Gray and her sister, who provided them to CNN.

Marcee Gray's father, Charles Polhamus, told CNN she spoke with a school counselor for about 10 minutes.

"The counselor said, 'Well, I wanted to let you know that earlier this morning one of Colt's teachers had sent me an email that said Colt had been making references to school shootings,'" Marcee Gray said in an interview with ABC News that aired Tuesday, recalling that conversation.

"I told them it was an extreme emergency and for them to go immediately and find Colt to check on him," Marcee Gray later said in a text message to her sister. "I don't understand what took them so long."

This is a developing story and will be updated.

CNN's Isabel Rosales reported from Atlanta while Dakin Andone reported and wrote this story in New York.

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