The disturbed conspiracy theorist accused of decapitating his dad and then holding up the head in a sick online video had first shot him dead with a gun he bought legally the day before, officials in Pennsylvania said Friday.

Justin Mohn, 32, bought the 9mm Sig Sauer handgun at a store a few miles from his Levittown home on Monday — hours after giving up a medical marijuana card so he could get the weapon legally, District Attorney Jennifer Schorn said.

The next day, he shot his 68-year-old father, Michael Francis Mohn, in the head, before decapitating him with a knife and a machete — and posting a rambling YouTube video holding up the plastic-wrapped head, Schorn said.

Mohn’s mom had come home to find her husband’s headless body Tuesday afternoon, with the head in a cooking pot, officials previously said.

The suspect was busted that night after breaking into a National Guard facility about 100 miles away — saying he was there to “mobilize the National Guard to raise arms against the federal government,” Schorn said.

He was allegedly still armed with the handgun — which was missing the one round used to gun down his dad.

The DA called it “horrifying” that the footage of him holding up his dad’s head had remained online for several hours, allowing at least 5,000 people to view it before it was yanked.

The video made it “evident to us that he was of clear mind in his purpose and what he was doing, aside from what his beliefs are,” Schorn said, saying the suspect had no history of mental illness.

Schorn declined to comment on the possible motive for the heinous murder. However, she noted that the rambling, 14-minute video featured threats to target and capture all federal employees — even sharing the address of a federal judge and putting a bounty on him.

Of his dad, who worked as an engineer with the geoenvironmental section of the US Army Corps of Engineers’ Philadelphia District, Mohn had said he “is in hell for being a traitor to his country,” Schorn said.

He also espoused a variety of conspiracy theories. He called himself the commander of America’s network of militias as he ranted against migrants, the Biden administration, the LBGTQ community, Black Lives Matter, and “far-left woke mobs,” while calling for the slaughter and public execution of FBI agents, IRS employees, US Marshals, federal judges, border control officers and others “betraying their country.”

Mohn was seen smirking in his mugshot as he was arraigned in the early hours of Wednesday.

A woman answering the phone at the Bucks County Office of the Public Defender said Friday that it was representing him and that the office declined further comment.

With Post wires