“Hopefully there will be some jihad on the anniversary of 9/11,” Goldberg reportedly told an FBI informant in a direct message.

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How Goldberg allegedly came to be radicalized — and adopt the personality of an extremist on the other side of the world — is not clear. But the complaint includes quite disturbing, lengthy exchanges between Goldberg, who authorities said went online under variations of the handle “AusWitness,” and the informant, identified only as “CHS.”

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The FBI became interested in AusWitness after someone using that name claimed responsibility for the attack at a “prophet Muhammad cartoon contest” in Garland, Tex., in May on a Web site.

“You might know me for inspiring the attacks in Garland … where two mujahideen entered an event mocking the Prophet Muhammad … with intent to slaughter,” the post said. The author lived in Perth, Australia, he said, and was a refugee from Lebanon “enamoured with the Islamic State’s ideology.”

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“The Jews are the worst enemies of Allah,” the post read. “When Islam conquers Australia, every single Jew will be slaughtered like the filthy cockroaches that they are.”

Informant CHS and AusWitness chatted in July. As AusWitness said he was “trying to get a mujahid in Melbourne to carry out Jihad, but he keeps delaying it,” the FBI linked his Twitter account to Goldberg’s residence in Florida. The conversation grew more serious in the weeks to come.

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CHS: “I don’t have any bombs. I don’t know how to make them.”

AusWitness: “What weapons do you have brother? I can send you guides on how to make bombs if you need help.”

The promised guidance, the complaint alleged, was sent. But where would the bombs be set off?

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AusWitness: “Have you decided what kind of attack to carry out on 9/11 … I was thinking a bombing. We could make pipe bombs and detonate them at a large public event.”

CHS: “I’m in the Midwest the closest place is Kansas City if you’re familiar.”

Kansas City was settled on.

AusWitness: “Where will the most people be in Kansas City on 9/11? That’s where we need to target.”

AusWitness found the “perfect place” — the Kansas City 9/11 Memorial Stair Climb, an annual event during which “343 Firefighters will embark on a 110 story climb to the top of the Town Pavilion high rise in downtown Kansas City in remembrance of the 343 firefighters killed on 9-11-2001,” as the event’s Web site explained.

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AusWitness: “Be careful … When you go there to place the bomb, make sure the bomb is VERY well hidden.”

CHS: “Where do you think would be best near the fire fighters or the crowd?”

AusWitness: “Good thinking … put the backpack near the crowd.”

AusWitness’s further advice for the pressure-cooker bomb: use “metal and nails” and “dip the screws and other shrapnel in rat poison before putting them in.” Those hit by them “will be more likely to die.”

Goldberg was arrested at home on Wednesday. He “claimed that he intended for the individual to either kill himself creating the bomb or, if not, that he intended to alert law enforcement just prior to the individual’s detonating the bomb, resulting in … credit for stopping the attack,” the complaint read.

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The FBI also received information from the Australian Federal Police (AFP) that someone with usernames linked to Goldberg had made comments about “pretending to be ISIS.” An AFP informant described Goldberg as an “online troll.”

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“These guys are … keyboard warriors,” Goldberg allegedly said online when asked if he feared his comments would result in an actual attack.

Goldberg’s family expressed surprise at the charges. His father told First Coast News he was “shocked.”

“We have no information to give you,” Frank Goldberg said.

A neighbor who spoke to Fox News described the family’s son as a bit of a recluse.

“They are active in the neighborhood association and their other kids come to community events, but the older son has never even walked down the street that I’m aware of,” Al Keene said. “I’m not surprised at all if his parents didn’t know anything about this. They seem like a great family.”