Police bolstered their presence at high-profile locations. Extra officers with bulletproof vests, rifles and dogs were assigned to spots such as Grand Central Terminal in New York. Plainclothes officers handed out fliers at a nearby hotel with a warning in large block letters: "If you suspect terrorism, call the NYPD."

The warnings come amid an investigation centering on Najibullah Zazi, a 24-year-old Denver airport shuttle driver who authorities say received al-Qaida explosives training in Pakistan and was found entering New York City two weeks ago with bomb-making instructions on his computer.