There are new concerns about potential terror attacks on stadiums, hotels, and other places where large numbers gather.
The concerns are reflected in a new warning from federal counterterrorism officials to police around the country.
The memos do not mention the ongoing high-profile investigation of a possible terrorist plot involving a Colorado man and associates in New York City. Instead, they describe the long-standing interest of terrorists to use homemade backpack bombs, car bombs, or even airplanes to attack such crowded public places.
And as a terrorism probe continues, New York City's transit agency says it has boosted police presence at some "key locations," including major train terminals.
The Metropolitan Transportation Authority says there's no credible threat to the city's subway system and commuter trains. But extra officers with helmets and bulletproof vests are at spots like Grand Central Terminal in midtown Manhattan.