The 14-year-old student who shot and killed two classmates and two teachers at his school in the US state of Georgia had sent an apology text message to his mother before opening fire, his grandfather told the New York Post.

The text message prompted the mother to make panicked calls to the school, apparently in an attempt to prevent the tragedy, he said.

Charles Polhamus said his daughter Marcee Gray was at his home when she received a text message from her son Colt Gray. “I’m sorry, mom,” it read.

After reading the message, she frantically called the school and tried to warn them of an “extreme emergency,” her relative told The Washington Post.

“I was the one who notified the high school counselor. I told them it was an extreme emergency and to go and find (my son) immediately, to check on him,” she told her sister.

Fearing the worst, she set off for school. Halfway there, she learned that four people had been killed, her father said.

“It’s horrible. It’s absolutely horrible,” Marcee told the New York Post.

After their panicked call, school officials attempted to avert the disaster. A school official also went to Colt’s classroom, but did not enter until minutes after he had left.

Both Colt and his father are now charged with murder. Authorities said the father gave him the AR-15 rifle used in the shooting as a Christmas present.