NEW YORK -- The government expanded a terrorism warning from transit systems to stadiums, hotels and entertainment complexes as investigators searched for more suspects Tuesday in a possible al-Qaida plot to set off hydrogen-peroxide bombs hidden in backpacks.
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.
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.
Two law enforcement officials speaking on condition of anonymity told The Associated Press that more than a half-dozen people were being scrutinized. The FBI said "several individuals in the United States, Pakistan and elsewhere" are being investigated.
"There's a lot more work to be done," said Police Commissioner Raymond Kelly, cautioning that the probe was still in its early stages.
In two bulletins sent to police departments Monday, 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 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 bulletins came just days after similar warnings about the vulnerability of the nation's mass transit systems and the danger of hydrogen peroxide-based explosives.
In a statement, the FBI and Homeland Security Department said that while the agencies "have no information regarding the timing, location or target of any planned attack, we believe it is prudent to raise the security awareness of our local law enforcement partners regarding the targets and tactics of previous terrorist activity."