Officials sent two bulletins to police departments Sept. 21, identifying no specific terrorism plots but urging law enforcement and private companies to be vigilant. The bulletins specified stadiums, entertainment complexes and hotels as potential targets. The recent flurry of terror attention involves a possible bomb plot in Denver and New York.

“While DHS and FBI have no information regarding the timing, location or target of any planned attack, we believe it is prudent to remind transit authorities to remain vigilant,” Homeland Security spokesman Sean Smith said.

Separately, law enforcement officials said a Colorado man may have been planning to detonate backpack bombs on New York City trains.