A Utah teenager “methodically” murdered his mom and three siblings — only getting stopped when his dad wrestled away the gun when he tried to kill him, too, according to prosecutors, who revealed disturbing new details this week about the five-hour bloodbath.

Colin “CJ” Haynie, 16, is being charged as an adult for the murders of his mother, brother and two sisters at their home in the small town of Grantsville, west of Salt Lake City.

“This is something that methodically happened over a five-hour period,” Tooele County Attorney Scott Broadhead said at a news conference Wednesday.

“This wasn’t a one-moment snap,” he stressed, saying the boy confirmed the killings last Friday before he stopped talking to investigators.

“We all have the same question: Why?” Broadhead said.

Each family member was shot shortly after entering the home Friday, Broadhead said.

Mom Consuelo Alejandra Haynie, 52, was first to be killed alongside 12-year-old daughter Milan, before 15-year-old Alexis was shot as she returned home about an hour later. The suspect’s 14-year-old brother, Matthew, was killed by a single gunshot to the head when he came home in the early evening, prosecutors said.

Dad Colin Haynie, 50, was the last to come home, and managed to wrestle a handgun from his son after getting shot in the leg, prosecutors said in court documents.

There was also a fifth child seemingly not involved in the killing spree, according to an obituary for Alejandra Haynie and the children.

“CJ” Haynie was charged with four counts of aggravated murder, one count of attempted aggravated murder and five counts of felony discharge of a firearm.

The teenager is scheduled to make his initial court appearance Monday.

With Post wires