Justin Mohn, 32, was arrested hours later Tuesday after breaking into a state National Guard facility about 100 miles away, armed with a 9 mm Sig Sauer handgun that was missing one round, prosecutor Jennifer Schorn said.
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Mohn told authorities he'd gone to the military installation "in an effort to mobilize the PA National Guard to raise arms against the federal government," Schorn said. His father, 68-year-old Michael Mohn, had worked for the US Army Corps of Engineers.
Officials give update on Bucks County beheading case
Justin Mohn also indicated he wanted to speak with Pennsylvania Gov. Josh Shapiro to "join forces," Schorn said.
According to an autopsy report, Michael Mohn suffered a gunshot wound to the head and was subsequently beheaded using a knife and machete.
Justin Mohn bought the Sig Sauer 9mm handgun the day before the murder in Bristol Township, Pennsylvania, Schorn said. Authorities say Mohn even gave up his medical marijuana card days before in order to make that purchase.
Schorn said Justin Mohn has "no history of diagnosed mental health issues." She went on to say Mohn does not presently have any record of voluntary or involuntary commitment for inpatient psychiatric treatment.
"It was 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.
She also noted that he is currently unemployed but previously lived in Colorado while working for a contractor for Microsoft. He was a graduate of Neshaminy High School and Penn State.
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His father was found dismembered inside his home in Bucks County on Tuesday.
"The call to emergency dispatchers came from the victim's wife. When officers arrived, they located the male deceased in the bathroom," the prosecutor's office said in a statement. "The victim's adult son was not present when police arrived and left the area in his father's vehicle."
Justin Mohn then went on social media afterward, posting a gruesome video on YouTube while going on an anti-government rant.
Why was a gruesome YouTube video of a decapitated head left online for hours?
In the more than 14-minute-long video, the man identifies himself as Justin Mohn and apparently reads from a written statement and at one point holds up what appears to be a bloodied head inside a clear plastic bag.
During the online diatribe, Justin Mohn railed against the Biden administration and the border crisis while declaring himself the new acting U.S. president under martial law and decrying his father because he was a federal employee.
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He also allegedly ordered that federal law enforcement officers be captured and tortured for information before being publicly executed.
Schorn said Justin Mohn also divulged the home address of a federal judge and "put a bounty on his head and the heads of other high-ranking officials."
She said he demanded violence against all federal employees while exempting state governors and state employees.
YouTube said the video was removed for violating its graphic violence policy and Justin Mohn's channel was terminated. However, it was viewed more than 5,000 times before it was taken down.
Neighbors say they noticed changes in Justin Mohn's behavior several months ago. While they say it was nothing violent, it was enough to make them uncomfortable. One said he was "strange," while another described him as "unhinged."
Sources tell ABC Philadelphia affiliate WPVI that federal authorities already investigated Mohn as recently as six months ago, after he allegedly made a threat against a U.S. District Court Judge. The case was closed that same month.
He has been charged with first-degree murder and abuse of a corpse, among other charges, according to police.
The CNN Wire and the Associated Press contributed to this report.