Annie Brown told the Washington Post that her sister, Colt Gray's mother, texted her saying she spoke with a school counsellor and urged them to "immediately" find her son to check on him.
Brown provided screen shots of the text exchange to the newspaper, which also reported that a call log from the family's shared phone plan showed a call was made to the school about 30 minutes before gunfire is believed to have erupted.
Brown confirmed the reporting to the Associated Press on Saturday in text messages but declined to provide further comment.
Colt Gray, 14, has been charged with murder over the killing of two students and two teachers at Apalachee High School in Barrow County, outside Atlanta, on Wednesday.
His father Colin Gray is accused of second-degree murder for providing his son with a semiautomatic AR 15-style rifle.
Their lawyers declined to immediately seek bail during their first court appearance on Friday.
Investigators previously interviewed the suspects
The Georgia teenager had struggled with his parents' separation and taunting by classmates, his father told a sheriff's investigator last year when asked whether his son posted an online threat.
"I don't know anything about him saying (expletive) like that," Gray told Jackson County sheriff's investigator Daniel Miller, according to a transcript of their interview obtained by the AP.
"I'm going to be mad as hell if he did, and then all the guns will go away."
Jackson County authorities ended their inquiry into Colt Gray a year ago, concluding that there was not clear evidence to link him to a threat posted on Discord, a social media site popular with video gamers.
The records from that investigation provide at least a narrow glimpse into a boy who struggled with his parents' breakup and at the middle school he attended at the time, where his father said others frequently taunted him.
"He gets flustered and under pressure. He doesn't really think straight," Colin Gray told the investigator on May 21, 2023, recalling a discussion he had had with the boy's principal.
