In one phone call, the undercover agent said McDowell wanted him to get the gun because he meant to conduct the attack outside Horry County, where he lived.
Benjamin Thomas Samuel McDowell, 29, of Conway, is charged as a felon in possession of a firearm or ammunition, according to a federal complaint.
McDowell has served multiple prison sentences in SC for various criminal offenses.
On Jan. 25, McDowell posted a message on Facebook urging people to be a fanatic for the white race and unite to clean up the world.
In another alleged conversation, McDowell said "I just be plotting it out, like, I mean you just run up there on them if they back there partying, and all, with a f****** AK and rip them sumbitches down, and throw, a damn, something at them".
According to an affidavit from the FBI that was filed in federal court, Mr McDowell had told an undercover FBI agent he was interested in "doing something on a f***ing big scale" and writing "in the spirit of Dylann Roof" on the wall of the building where he would commit the killings.
The document, which is in the public domain, states that Mr McDowell had "established White Supremacy Extremist connections while serving prison sentences in SC for various criminal offenses", adding that his "tattoos on his extremities indicated his affiliation with the WSE (white supremacist extremists)". I want to do that s***.
In late January, McDowell wrote in a Facebook post that he wished white supremacists would be more like Dylan Roof.
McDowell was already a felon on unrelated charges and had spent time in jail, NBC News reported.
McDowell met with the undercover agent one week later, believing the agent was someone who "handled problems for the Aryan Nations". He proclaimed that he had the heart for such action "but I don't have the good training".
A message sent from his Facebook messenger account the next day showed a request for an "iron", a coded term for a gun. Then McDowell revealed why he needed a gun. An FBI undercover employee met with McDowell less than a week later to discuss the firearm purchase.
At the time of his arrest, McDowell was in the parking lot of a Myrtle Beach Hampton Inn holding a bag.
On Wednesday, McDowell handed the operative $109 for hollow point bullets and a.40-caliber Glock pistol that had been disabled without his knowledge.
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