Chilling new details emerged Sunday in the plot to attack Big Apple synagogues, including how the mastermind told cops he runs an online white supremacist group and has Nazi memorabilia because “it is really cool,” court documents allege.

Christopher Brown, 21, who tweeted anti-Semitic threats with the username VrillKhan and under the handle @VrillGod, told cops he and accused cohort Matthew Mahrer met at St. Patrick’s Cathedral in Manhattan on Friday to carry out the attacks before they were busted.

Police said the pair had purchased a Glock 9mm handgun for $650 in Pennsylvania.

“I have a sick personality,” Brown told police, according to a criminal complaint.

“I was going to be a coward and blow my brains out with it,” he allegedly told cops. “It took me three years to finally buy the gun. Matt is one of my followers. I have Nazy paraphernalia in my house. I think it’s really cool.

“I also operate a white supremacist Twitter group,” the complaint quoted him saying.

The NYPD had intercepted a series of threatening tweets from Brown starting with a Nov. 12 boast that, “Big moves being made on Friday.”

Four days later, Brown allegedly posted a 6-second video with a chilling message: “Brutally murder people with hatchets and whatnot. How it feels to kill people.

“Gonna ask a Priest if I should become a husband or shoot up a synagogue and die,” he allegedly posted Thursday.

“This time I’m really gonna do it,” cops said he posted Friday.

Police recovered the gun and a bulletproof vest from Maher’s Upper West Side apartment after they arrested the plotting pair.

“The two defendants possessed a firearm, a high-capacity magazine, ammunition, an [8-inch] long military-style knife, a swastika arm patch, a ski mask and a bulletproof vest, along with other things,” Manhattan District Attorney Alvin Bragg said in a statement.

“Hateful anti-Semitic targeting of synagogues is deplorable,” Bragg said. “The Manhattan DA’s office will now pursue accountability and justice in this case with the full resources of our counter-terrorism problem and recently enhanced and expanded hate crimes unit.”

Brown was arraigned early Sunday and charged with making a terroristic threat and six weapons possessions charged, according to court records. He was held without bail.

Maher, 22, was arraigned Saturday on weapons possession charges and ordered held on $150,000 bail or a $300,000 bond despite a request by the Manhattan DA’s office that he be held without bail in the case.