A 14-year-old boy accused of shooting dead his entire family "isn't bothered" by the horrific tragedy, authorities say.

Mason Sisk, 14, allegedly shot his parents and three young siblings aged just six, five and six-months-old in September 2019.

Sisk is said to have gunned down his relatives when he discovered his mother was actually his stepmum, but investigators in Alabama, US, have yet to confirm a motive.

Prosecutors say the teen shot his dad, John Sisk, 38, and stepmom Mary as well as his three young siblings, Kane, Coleson and Aurora before calling 911 and telling officers he had been in the basement of his home when he heard gun fire.

Siblings, Kane, Coleson and Aurora were killed

Limestone County chief juvenile probation officer Tara Pressnell said: "Mason does not seem bothered by the fact that he is accused of murdering his family".

“He has not shown any sign of remorse. While in detention, he has not talked about his family at all," Pressnell added.

A cousin told reporters that in the weeks prior to the alleged shooting Sisk had shown "problem behaviour at school" and burnt animals alive.

Mary Sisk, 35, was the boy's step mother

But when officers confronted Sisk with inconsistencies in his statement, he admitted to shooting all five people, authorities say.

Sisk has been charged with five counts of capital murder and faces trial as an adult despite his young age.

Father John Sisk was also among the victims

He was moved to Limestone County Jail after Juvenile Judge Matthew Huggins said last week there were "no reasonable grounds" he was mentally ill.

Under a recent change in the state’s law, Sisk can only be charged to life imprisonment.?

But he can’t be given the death penalty since he’s under the age of 16.