Aaron Greene, 31, son of the president of an artwork-restoration business, and his girlfriend, Morgan Gliedman, 27, daughter of a prominent oncologist who gave birth after her arrest, were charged in December with criminal possession of a loaded firearm and criminal possession of an explosive substance.
Authorities said investigators seized 7 grams of hexamethylene triperoxide diamine, an explosive chemical, at the couple's Greenwich Village apartment.
Police also found two shotguns -- one with the barrel sawed off -- dozens of shells, a flare launcher with four flares and nine high-capacity rifle magazines, authorities said.
Acquaintances of Greene told police they saw him sprinkling white powder -- presumably HMTD -- on the ground in Washington Square Park and then striking it with a rock to cause an explosion, The New York Times reported Wednesday.
Meanwhile, police confiscated 21 guns, brass knuckles, two stun guns and a switchblade from the home of Greene's friend, Daniel Whittaker, a corrections officer. Greene had told police he gave some of his weapons to Whittaker to hold for him.
Police said they found undated letters by Greene at Whittaker's home in Orangeburg, N.Y., in which he scrawled the words "kill" and "kill them all" before signing off with a lightening-bolt symbol associated with Adolf Hitler's SS organization, the Times reported.
Greene does not appear to be active in any political movements, police told the Times.
New York City police spokesman Paul J. Browne called the couple "admitted heroin addicts."