Among the buildings on the block is one that serves as a local headquarters for the Hell's Angels biker gang. Investigators wouldn't say if the gang is a target of suspicion.
The plastic explosive, officially called "Composition 4" was discovered by a volunteer who was cleaning up debris in the Marble Cemetery over the weekend.
NYPD Commissioner Raymond Kelly said the C-4 had gone unnoticed in the graveyard for more than a year after the cemetery caretaker dug it up while planting bushes in May or June of 2009.
"I'm not certain if he knew what was in the bag, but he takes the bag and puts it next to a tree and all indications are that bag stayed at the location until yesterday," Kelly said.
Police say the C-4 was not an immediate danger to the public because it was not accompanied by a detonating device.
Investigators are also looking into 2 suspicious, notes left near the Marble Cemetery. One of them was a rambling text from a writer who claimed to be Jesus Christ. The other note was a message scribbled on the 2nd Street sidewalk that read: "I hope one of you finds this."