Terror warning expanded to hotels, stadiums

The warnings come amid an investigation centering on Najibullah Zazi, a 24-year-old Denver airport shuttle driver who authorities say received Al Qaeda 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 cellphones 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’’ are being investigated.

“There’s a lot more work to be done,’’ said Police Commissioner Raymond Kelly.

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.

In Boston, Police Commissioner Edward Davis said in a statement yesterday that there is no known specific threat to Boston or Massachusetts. However, Boston police officers are routinely reminded to remain vigilant, he said.

The department reminds community members to report any suspicious behavior to the by calling 911 or the Crimestoppers tip line, 1-800-494-TIPS.

The bulletin on stadiums noted that an Al Qaeda 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, saying that terrorists have used such places to build bombs.