Targets of a possible bomb plot may include stadiums and hotels, federal officials say.
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 in New York bolstered their presence at high-profile locations. Extra officers with bulletproof vests, rifles and dogs were assigned to spots such as Grand Central Terminal. Plainclothes officers handed out fliers at a nearby hotel with a warning in large block letters: "If you suspect terrorism, call the NYPD."
Authorities in the Twin Cities said they don't expect to bump up security in light of the warning because no specific threats have been identified.
The warning comes 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 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, said that 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" were being investigated.
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 to be on the lookout for any possible bomb-making activity at self-storage facilities.
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."
"We have no specific, credible threat against mass transit here," said Metro Transit spokesman Bob Gibbons. He said passengers should be vigilant and report "anything they see that makes them uncomfortable, such as left-behind luggage or backpacks."
The Mall of America will continue to observe its standard security measures, which include three trained dogs, said spokesman Dan Jasper. Minneapolis police spokesman Sgt. William Palmer said his department will increase security only if a threat is identified.
Staff writer Chao Xiong contributed to this report.