updated 10/11/2010 4:39:28 PM ET 2010-10-11T20:39:28

A caretaker doing gardening work at a historic cemetery dug up a plastic garbage bag containing military-grade explosives last fall and left it at the site, where it remained until a volunteer told authorities about it Monday, setting off a big police response.

The employee found the C-4 last year after digging down about a foot into the ground at New York City Marble Cemetery on Manhattan's Lower East Side, Police Commissioner Raymond Kelly said. It contained eight sticks of the explosive, but Kelly said they couldn't have gone off because there were nothing to detonate it.

It was unclear how long the bag had been at the cemetery, but "we believe it's been there for a significant period of time," Kelly said. He said it appeared to be military-grade explosive similar to the material used in the 2005 London transit bombings, but that there were no suspicions of terrorism in connection to the discovery.

The caretaker left the bag on the property, by a fence in the back. A volunteer came across it over the weekend, and initially also left it there before calling police on Monday. Authorities closed down nearby streets as they investigated.

C-4 is a plastic explosive that is more powerful than TNT. It's commonly used by the military, because it is easy to shape and relatively hard to set off by accident. C-4's main ingredient is RDX, which is also used in fireworks.

It is relatively insensitive to impact, friction or fire, although large quantities can explode if burned. Even shooting it with a rifle won't trigger the reaction. Only a detonator or blasting cap will do the job properly. Less than a pound of C-4 could potentially kill several people, and several blocks of C-4, weighing about 1.25 pounds each, could potentially demolish a truck.

Kelly said the material from the cemetery was being taken to the police range where explosives are tested. Authorities also were digging around in the cemetery to see if any more material was found.

A call to the cemetery seeking comment was not immediately returned.

Police were also looking into two messages that were found in the area to see if there was any connection. One, written in chalk on the sidewalk near the cemetery, said, "I really hope one of you finds this." The other, a note placed on a police car at the precinct near the site, made a reference to Jesus Christ being kept out of the neighborhood, and was signed by someone identified as "Jesus Christ." 

The Associated Press contributed to this report.

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