Top NYPD brass including Commissioner William Bratton held an emergency meeting Tuesday to discuss the threat and their response to it, sources said. Few details were released about the nature of the threat.

A federal law enforcement source said the threat is not specific and could take place in several major U.S. cities.

All New York City police officers received a bulletin Tuesday afternoon outlining the department’s tactical plan and warning officers to stay vigilant, according to sources. That internal memo mentioned social media being used as a tactic and that a possible attack could come without warning.