He did not elaborate. He rarely does. He is famously anonymous, putting up his distinctively playful stencils unnoticed, and there was no response on Tuesday to an e-mail to a London publicist who represents him. He did not even say exactly where the twin towers silhouette was, but others soon pinpointed the location, again on Instagram and Twitter.

That made things easy for Jane Viera, a publicist from Los Angeles who is on vacation in Manhattan. “They said ‘Jay and Staple,’ and we were like, ‘We’re there,’ ” she said, referring to the location of the silhouette, on the Staple Street side of the building at 9 Jay Street, about half a mile from where the World Trade Center stood.

Some in the crowd discussed blog posts about other Banksy works that have appeared in recent days. They wondered how long this one would last before being tagged over, or damaged. They said they had heard stories of locals who commandeered a site, covering the Banksy and making visitors pay to see it.

That did not happen at midday on Staple Street. The crowd took photos of the twin towers silhouette, some with cellphones, some with digital cameras. Some trained their eyes on others in the crowd — and up, at the windows of surrounding buildings — because they figured Banksy was somewhere nearby. They said true artists attend their own shows.

Oscar Salguero, who designs children’s products, wondered whether the flower was Banksy’s, or had been put there by someone else. Either way, he said, the flower “looks like flames.”