It's not clear whether the intercom was turned on purposefully to alert the school's staff to the menace or whether the intercom was on for morning announcements.
Either way, it caught the initial moments of Adam Lanza's lethal fury and gave teachers and others life saving moments to lock their doors and try to hide their children.
Among Lanza's first victims was school principal Dawn Hochsprung, 47. Third grader Tori Chop said that she could hear her principal's final moments as Lanza barged in with his weapons.
"Yeah, yeah, she was crying. I thought she was screaming," Chop told "Good Morning America." "That's what we heard over the loudspeaker. We heard kids crying."
Instead of the morning announcement, they listened to murder.
"We kept hearing gun noises and 'put your hands up'…we kept hearing that," said Chop.
Chop's teacher Teri Alves, like other staff in the school, sprang into action.
"She went out to the door, she locked the door and taped a piece of white tape over the window of the door," Chop said of her teacher. "And then she told us to go in the corner."
As the students listened to the rampage unfold, Alves tried to sooth them.
"She's just saying it's going to be okay," Chop said. "Just be quiet."
"We were in the corner all scrouched in, and all the girls were crying, and a couple of boys. The boys had their eyes wide open, watching everything," she said.
Lanza killed 20 children and six adults, including Hochsprung, before shooting himself. Lanza's mother was also found murdered in their home.
Chop believes it was her teachers' swift reaction to the broadcast that kept her and her classmates alive and wants to tell Alves she is grateful.
"I want to say, you saved my life. There's 19 people in our class," Chop said. "And she saved all of us."