THE dad of a teenage school shooter has been arrested after his sad attempts at bonding with his daughter ended in carnage.

Disturbing new details have emerged about what led up to Natalie Rupnow, 15, killing two at Abundant Life Christian School in Madison, Wisconsin, before turning the gun on herself.

Investigators are unraveling the truth behind the shooter’s tumultuous relationship with her father, and how she brought a stash of weapons into the school on December 16.

That day, the teen snuck a 9 mm Glock handgun and a .22-caliber Sig Sauer pistol with magazines and extra ammo into her bag and opened fire on classmates in a study hall.

She murdered teacher Erin Michelle West and classmate Rubi Bergara, 14, and hurt seven others before killing herself.

She donned a black T-shirt with a bullseye emblazoned on the front for the attack.

Jeffrey revealed that he had purchased both weapons for his daughter as presents because he thought that shooting was a way for the two to bond, Wisconsin prosecutors said in a complaint unsealed Thursday.

Rupnow had spiraled into a mental health crisis since Jeffrey and her mom Melissa Rupnow separated, and struggled with self-harm and thoughts of suicide.

Jeffrey said that her cutting had gotten so severe that he had to lock up all of their knives.

Rupnow was in therapy to try to help her socialization skills. Her therapist said that the teen was suffering from post-traumatic stress disorder from the divorce, Melissa told detectives.

Jeffrey said that he took his daughter shooting about two years before she opened fire on the school, and she appeared to enjoy herself.

He saw the activity as a sick way to bond, but was shocked when her interest in guns “snowballed” out of control, he told cops.

Despite her obvious signs of struggling, Rupnow told his daughter that the password to his gun safe was the last four digits of his Social Security number backward.

Now, he’s been charged with two counts of intentionally giving a dangerous weapon to a person under 18, causing death, and contributing to the delinquency of a child.

He was arrested on Thursday, and his first court appearance is scheduled for Friday.

HAUNTING SIGNS

When detectives searched Rupnow’s room, they uncovered a disturbing note titled War Against Humanity where she described humanity as “filth.”

At one point, she blasted people who “smoke their lungs out with weed or drink as much as they can like my father.”

The killer also revealed that she was able to secure the guns “by lies and manipulation, and my fathers stupidity.”

Cops discovered a handwritten schedule explaining how she planned to start her shooting at 11:30 am and kill everyone on the first and second floors of the school by 11:55 am.

They also found a cardboard model of the building and maps of the school.

Rupnow was active on social media, and had connected with people online over their admiration of school shooters.

After the horrific attack, Alexander Paffendorf, 20, was arrested at his family home in California for allegedly planning attacks on government buildings with Rupnow, according to court filings.

He’s now been hit with a three-year gun violence restraining order, which bars him from purchasing firearms.

FATHER’S FEARS

Jeffrey appeared to joke about his daughter killing him before her murderous final act.

Around 10 days before the shooting, he sent a text to a friend saying that Rupnow would shoot him if he left “the fun safe open right now,” prosecutors said.

One of Rupnow’s friends said that Jeffrey was “frequently verbally aggressive” and that his daughter had described him as a “drinker,” according to the complaint.

After the shooting, the dad warned other parents about gun safety, and said that they should change the passwords to gun safes every three months.

“Kids are smart and they will figure it out,” he said.

“Just like something trying to hack your bank account. I just want to protect other families.”

DAD CHARGED

Jeffrey was arrested on Thursday and was scheduled to make his first court appearance on Friday.

According to acting Madison Police Chief John Patterson, he has been cooperative throughout the investigation.

Jeffrey isn’t the first parent of a school shooter to face charges.

Colin Gray, the father of suspected Apalachee High School shooter Colt Gray, was hit with murder charges after his son used his guns to allegedly kill four people.

And last year, the mom of convicted school shooter Ethan Crumbley, who murdered four students, was found guilty of involuntary manslaughter for ignoring her son’s cries for help.