New York ( CNN ) -- A man arrested on suspicion of possession of an explosive substance allegedly told people he planned to blow up the iconic arch in New York City 's Washington Square Park , authorities say . Aaron Greene , 31 , was arrested in December after police say they discovered the explosive substance and a sawed-off shotgun in his Manhattan apartment while serving an unrelated warrant for felony grand larceny against his girlfriend , Morgan Gliedman . As part of the investigation , New York Police Department Intelligence Division detectives found witnesses who recounted alleged conversations that Greene had with them regarding the arch , Deputy Police Commissioner Paul Browne told CNN on Thursday . `` I 'm making bombs , '' Greene allegedly told witnesses , according to Browne . `` What for ? '' the witnesses asked , the deputy commissioner said . `` To blow up the arch , '' Greene allegedly replied , Browne said . Formally known as the Washington Square Arch , it was built in the early 1890s and is modeled after the famed Arc de Triomphe in Paris . It was formally dedicated at the park in 1895 , and today it is considered one of Greenwich Village 's iconic landmarks . Greene , according to Browne , tested a small amount of the explosives -- believed to be hexamethylene triperoxide diamine , or HMTD -- in the park to demonstrate the power of the explosives to the witnesses . During the test , Browne blew up a small amount of the chemical by hitting it with a rock , which caused it to ignite and cause a small explosion , Browne said . Police issued no public reports of an explosion at the park at the time witnesses said the incident allegedly occurred . A week ago , law enforcement sources speaking on condition of anonymity said Greene allegedly set off a small explosion in front of witnesses inside his apartment . That 's what initially led witnesses to alert the NYPD that Greene possessed explosive chemicals , said the sources , who were not authorized to release details of the investigation to the media . After Greene was arrested , he was questioned by police about whether he had other weapons . He allegedly told police `` he had given them to a friend in law enforcement , '' Browne said . Police say Greene did not identify the so-called friend , but investigators identified him as Daniel Whittaker , 33 , in Orangeberg , NY . Authorities say they discovered a cache of firearms in Whittaker 's home . `` We found 21 guns , including an Uzi and assault weapon , a switchblade , brass knuckles and a stun gun , '' said Browne . Whittaker has not been charged by the NYPD , Browne said . Whittaker is a suspended corrections officer in Rockland County , NY , Browne said . Prosecutors say he is currently on trial in Rockland County on charges of drug possession and fleeing an officer . His attorney in that case , Joey Jackson , said that while Whittaker knows Greene , he had no knowledge of Greene 's alleged criminal activities and was not guilty by association . Jackson is a CNN legal analyst . Investigators also say they discovered undated letters allegedly written by Greene at Whittaker 's home . `` In one , Greene says his parents have sent him to Greece because they 're concerned if he stayed in New York , he 'd kill 100 people , '' Browne said . `` On another piece of paper , he also wrote the word 'kill ' dozens of times and 'kill them all . ' '' Greene also drew a symbol , according to Browne , that was the same one used by the Nazi Schutzstaffel , commonly known as the `` SS '' . `` It looks like two lightning bolts , '' he said . Police say Greene was arrested on December 29 after they discovered a plastic container with seven grams of the explosive in the apartment he shared with 27-year-old Gliedman . In addition officers also found the sawed-off shotgun and multiple rounds of ammunition , police said . `` The biggest cause of concern is the explosive , '' Police Commissioner Ray Kelly told CNN . Police said they also discovered what they described as manuals about how to make explosive materials and bombs , including a collection of pages with a cover page titled `` The Terrorist Encyclopedia . '' Greene and Gliedman were each charged with felony criminal possession of a weapon in the first degree and second degrees , the district attorney 's office said , adding that when the charges against the pair may be amended . Gliedman , who was pregnant at the time of her arrest , also was charged with felony grand larceny involving credit card theft . Shortly after her arrest , Gliedman went into labor and gave birth to a daughter who she named Melody , Browne said . Gliedman was released Tuesday on $ 150,000 bond , according to the district attorney 's office . Greene is being held without bond . The pair are scheduled to be arraigned in a Manhattan court on January 29 . Greene 's attorney , Lisa Pelosi , and Gliedman 's attorney , Gerald Shargel , both declined a CNN request Thursday for comment . CNN 's Ross Levitt contributed to this report .
Greene , December 29 , was arrested by police on 31 . He was initially charged with possession of an explosive substance . Police say they have evidence that Aaron Greene allegedly planned to target a monument . He allegedly told people he wanted to blow up Whittaker , police said .