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.