Police rushed to the Elkmont home in Alabama where they found an adult and two children dead at the scene.
Another adult and child were airlifted to hospital in critical condition but later died from their injuries.
However, just hours after the phone calls were made the boy confessed to police that he was the one who shot his family dead before disposing of the gun.
The family members have since been identified as the teen's father John Sisk, 38, stepmother Mary, 35 and his three siblings aged six, five and six-months-old.
His motive is unknown at this stage.
The mass shooting has sent a wave of heartbreak throughout the town, with local Mayor Tracey Compton describing the incident as an 'absolutely terrible thing'.
'Our heartfelt condolences are extended to the family, the school, the whole community,' she said.
According to gun violence research group, Gun Violence Archive, there have been more mass shootings in the US in 2019 than there have been days in the year.
As of September 3, the nonprofit organisation reported that there had already been 289 incidents.
The news follows an announcement by retail chain Walmart that they will reduce gun and ammunition sales and cease the sales of handguns in Alaska.