A man who was arrested last month on a charge of possession of a chemical explosive in his Greenwich Village apartment recently told acquaintances that he was busy “making bombs” to “blow up” Washington Arch, a police official said Wednesday.

In an interview with investigators, the acquaintances said they witnessed the man, Aaron Greene, sprinkling a white powder on the sidewalk inside Washington Square Park and then striking it with a rock, triggering an explosion, according to Paul J. Browne, the Police Department’s chief spokesman.

Investigators believe the powder was a chemical called hexamethylene triperoxide diamine, or HMTD, an explosive that police officers said they found last month in the West Ninth Street apartment that Mr. Greene shared with his girlfriend, Morgan Gliedman, Mr. Browne said.