Some mothers spoke of wishing to avoid taking children on the subway. Other people talked about rejiggering appointments to keep from having to take the train at rush hour or to avoid going to Midtown Manhattan at all. Irrational gaming took hold — the R train was probably safer than the Nos. 2 and 3; it always seems less crowded. One longtime resident of Williamsburg posted on Facebook that she now felt uneasy in a neighborhood where she had always felt so safe. If, as in Paris, extremists were going to concentrate on harming the young and urbane, out enjoying stylish consumer pleasures, Williamsburg seemed to possess horrific potential as a focus of interest.

If your friends accused you of indulging an overactive neurosis, you could counter that ISIS has not seemed casual about New York. On Wednesday, a video surfaced that appeared to direct a threat at the United States, using imagery of Times Square and a man strapping explosives to himself. In interviews, Police Commissioner William J. Bratton pointed out that the film used old footage and was largely a piece of propaganda, but that the possibility of terrorism was being taken very seriously. Mayor Bill de Blasio stressed that there had been no real, specific threats to the city and that his administration had just established an elite counterterrorism force. The new team, called the Critical Response Command, is made up of more than 500 officers with special training; roughly 100 of them will patrol the city at any given time. Many more have been taught to handle rapid response to simultaneous attacks.

Since 9/11, the Police Department, in conjunction with federal and international investigators, has thwarted 16 known terrorist plots against New York City. The schemes included a planned cyanide attack on the subway system in 2003; an attack on the New York Stock Exchange and Citigroup headquarters in 2006; and planned suicide bombings of the PATH train along with the detonation of jet-fuel storage tanks at John F. Kennedy Airport, also in 2006. With approximately 35,000 officers, the police force is larger than some armies. Lydia Khalil, a former counterterrorism analyst for the department, said the division she worked in had only grown more sophisticated since she left in 2009.

Ms. Khalil also pointed out that the Muslim population of the United States is better off and better integrated than the Muslim population of Europe. “The thing about having an American identity is that it doesn’t force you to give up another identity,” she said. “I was born in Egypt, and I can retain an Egyptian identity here while still being an American. It’s very difficult within the European context to reconcile two identities.”