The school security guard was the first to respond to reports of danger over the school’s walkie-talkies. Someone asked if the loud sounds ringing though the school were firecrackers, according to May, who also carries a radio.

“I heard Aaron say, ‘No, that is not firecrackers,'” May recalled. “That’s that last I heard of him.”

The gunman on Wednesday allegedly triggered a fire alarm in an effort to draw staff and students out of their classrooms before unleashing a torrent of bullets. Fifteen people were killed in the shooting spree and another two died in the hospital. At least another dozen were injured, several of them critically.