NEW YORK - The government expanded a terrorism warning from transit systems to U.S. 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.
"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.
Two law enforcement officials speaking on condition of anonymity because they were not authorized to discuss details of the investigation said more than a half-dozen people were being scrutinized in the alleged plot.
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 and obtained by the AP, 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 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 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."
A half-dozen terrorism warnings and alerts have been issued in the past week amid the investigations in New York and Denver. Bulletins - particularly about hotels as possible terrorist targets - are common, and often don't make news. In fact, they are so common that many Americans in the past few years have accused Washington of fearmongering.
Some Americans were blase about the latest warnings.
"If it happens, it happens," said Lynn Calhoun, an Indianapolis computer programmer who visited Conseco Fieldhouse, the home of the NBA's Indiana Pacers, to buy a ticket for a Trans-Siberian Orchestra concert there in December. "Where are you going to go? What are you going to do? You can't just go and hide out in Canada for a month."
At Citi Field in New York, where the Mets were playing the Atlanta Braves on Tuesday, Dana Brand said he once canceled a trip because of a security warning, but "I found out later that it was all bogus."
So when it comes to the latest warning, the Newtown, Conn., resident said: "I don't pay any attention to it."
James Orash waited for a commuter train outside Camden Yards, Baltimore's ballpark, with his wife.
"If they're going to hit us, there, that's where they're going to hit us," Orash said, looking at the stadium. "They already took two buildings down once. Eventually, that's what's going to happen. If they hit us next time, it's going to be big."
New York's transit agency said it increased the police presence around the city. The vigilance is playing out during a meeting of the U.N. General Assembly, with Obama and other leaders from around the world in town. Also, thousands of policymakers and other visitors are arriving in Pittsburgh for a two-day economic summit of wealthy and developing nations.
New York's Police Department produced a 10-minute videotape it has begun showing at roll call instructing officers to be on the lookout for potential bomb-making ingredients. The video puts special emphasis on hydrogen peroxide - a common ingredient in homemade explosives - as well as cans of acetone and bags of ammonium nitrate.
Stadiums around the country provided few specifics about how they were responding but stressed that they have been vigilant ever since the Sept. 11, 2001, attacks.
"We're aware of the memo," said Bob Moore, spokesman for the Kansas City Chiefs, who play at Arrowhead Stadium. "It just underscores the high levels of security we've had and will continue to maintain. We've been in that mode for some time."
In New Jersey, home of Giants Stadium, the state homeland security office said there will be an increased police presence at key locations, random bag searches and greater use of surveillance cameras and undercover operations.
At MGM Mirage, which owns two arenas and the most casinos on the Las Vegas Strip, spokesman Gordon Absher said the alert "encouraged us to remain vigilant." No protocols were changed, he said.
Stuart Pringle, a 55-year-old businessman from Zimbabwe, was visiting Las Vegas for a week and said the security warnings wouldn't change his plans.
"We all live with some degree of risk in our lives," Pringle said. "This country can't afford to stop the quality of life that it provides. I have absolutely no fear."
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Associated Press writers contributing to this report include: Eileen Sullivan in Washington; Samantha Gross and Mike Fitzpatrick in New York; Don Mitchell and P. Solomon Banda in Denver; Alex Dominguez in Baltimore; Charles Wilson in Indianapolis; Beth DeFalco in Trenton, N.J.; Ashley M. Hehrer in Chicago; and Oskar Garcia in Las Vegas.