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, 24, a 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.
Zazi's arrest in Colorado last week touched off the most intense flurry of government terror warnings and advisories to come to light since President Barack Obama took office.
Though Zazi is charged only with lying to the government, law enforcement officials said he may have been plotting with others to detonate backpack bombs on New York trains in a scheme similar to the attacks on the London subway and Madrid's rail system. Backpacks and cell phones were seized in raids on apartments Zazi visited in New York.
"It's not totally clear to us at this point what it is they had in mind, though I think it is clear that something very serious and something very organized was under way," Attorney General Eric Holder told CBS.
The FBI said "several individuals in the United States, Pakistan and elsewhere" are being investigated.
"There's a lot more work to be done," Police Commissioner Raymond Kelly said, cautioning that the probe was still in its early stages.
Nashville police 'alert'In two bulletins sent to police departments Monday and obtained by The Associated Press, federal counterterrorism officials urged law enforcement and private companies to be vigilant at stadiums, entertainment complexes and hotels.
The bulletin on stadiums noted that an al-Qaida training manual specifically lists "blasting and destroying the places of amusement, immorality and sin ... and attacking vital economic centers." Counterterrorism officials are also advising police officers to be on the lookout for any possible bomb-making at self-storage facilities, noting that terrorists have used such places to build bombs.
"The police department has been very involved in the security, not only of LP Field, but Vanderbilt stadium," Metro police spokes man Don Aaron said Tuesday.
"Without going into specifics, there are a number of measures that have been taken over the years. We will continue to be very alert at these venues as is the Vanderbilt police at Vanderbilt stadium."
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