Counterterrorism officials have issued security bulletins to police around the U.S. about terrorists' desire to attack stadiums, entertainment complexes and hotels - the latest in a flurry of such internal warnings as investigators chase a possible bomb plot in Denver and New York.
In the two bulletins - sent to police departments Monday and obtained by The Associated Press - officials said they know of no specific plots against such sites, but urged law enforcement and private companies to be vigilant. These two bulletins followed on the heels of a similar warning about the .
The bulletin on stadiums notes that an al Qaeda training manual specifically lists "blasting and destroying the places of amusement, immorality, and sin... and attacking vital economic centers" as desired targets of the global terror network.
A joint statement from Department of Homeland Security and the Federal Bureau of Investigation said 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."
However, CBS News correspondent Bob Orr says the warnings, while legitimate, should not induce panic - they are routine advisories, and the government puts out numerous such alerts every month.
Officials also noted the law enforcement bulletins are not intended for the public.
However, a half-dozen alerts issued in the last week have received increased attention amid the current investigations in New York and Denver. The first of these, about hydrogen peroxide-based explosives, specifically referred to the investigation in New York.
Separately, law enforcement officials said a Colorado man may have been planning with others to detonate backpack bombs on New York City trains in a terrorism plot similar to past attacks on London's and Madrid's mass-transit systems.
New York City's transit agency increased police presence at "key locations" in light of a continuing terrorism probe, although it said that there is no credible threat to the city's subway system and commuter trains.
Police Commissioner Raymond Kelly said the city is "obligated" to put itself on heightened alert whenever possible plots surface because of previous attacks in the city.
Extra officers with helmets and bulletproof vests are at spots like Grand Central Terminal in midtown Manhattan.
Meanwhile, CBS News correspondent Bob Orr reports that the FBI is now pressing an urgent search for "a core group" of nine to 12 other people associated with Najibullah Zazi who may have knowledge of his plans.
Sources tell Orr that while some potential suspects have been interviewed and cleared, other names continue to surface as the FBI tries to fully understand Zazi's intentions.
In a statement, the FBI says that "several individuals in the United States, Pakistan and elsewhere" are being investigated.
Investigators say Zazi, a 24-year-old Afghanistan-born immigrant who is a shuttle van driver at the Denver airport, played a direct role in the terror plot that unraveled after an overnight 1,600-mile trip from Denver to New York City around the anniversary of the Sept. 11, 2001 attacks. He made his Monday and remains behind bars.
Zazi and two other defendants have not been charged with any terrorism counts, only the relatively minor offense of lying to the government. But the case could grow to include more serious charges as the investigation proceeds.
Arrest Affadavit: Najibullah Zazi
Arrest Affadavit: Mohammed Wali Zazi
Arrest Affadavit: Ahmad Wais Afzali
Backpacks and cell phones were seized last week from apartments in Queens, where Zazi visited. Orr reports that one cell phone found in Zazi's possession contained a video of New York's Grand Central Station, which prompted warnings about a possible mass transit attack.
CBS News learned Monday that despite the arrests, law enforcement officials still have "deep concern" that the government may have .
Zazi has publicly denied being involved in a terror plot, and defense lawyer Arthur Folsom dismissed as "rumor" any notion that his client played a crucial role.
But according to the complaint filed in the case, Zazi "received instruction from al Qaeda operatives on … weapons and explosives."
Informants inside Pakistan have told authorities they saw Zazi in an al Qaeda training camp during his admitted travels to the country's lawless tribal regions in the summer of 2008, reports CBS News chief investigative correspondent Armen Keteyian.
Publicly, law enforcement officials have repeatedly said they are unaware of a specific time or target for any attacks. Privately, officials speaking on condition of anonymity because they were not authorized to discuss the case said investigators have worried most about the possible use of backpack bombs on New York City trains.
In a bulletin issued Friday, the FBI and Homeland Security Department warned that improvised explosive devices are the most common tactic to blow up railroads and other mass transit systems overseas. And they noted incidents in which bombs were made with peroxide.
In that bulletin, obtained by The AP, officials recommended that transit systems conduct random sweeps at terminals and stations and that law enforcement make random patrols and board some trains and buses.
Investigators feared Zazi may have been involved in a potential plot involving hydrogen peroxide-based explosives, according to two law enforcement officials, who spoke on condition of anonymity because they were not authorized to discuss the investigation.
The FBI said they found notes on bomb-making instructions that appear to match Zazi's handwriting, and discovered his fingerprints on materials - batteries and a scale - that could be used to make explosives. He also made a trip to Pakistan last year in which he received al Qaeda explosives and weapons training, the government said.
Zazi, a legal resident of the U.S. who immigrated in 1999, told the FBI that he must have unintentionally downloaded the notes on bomb-making as part of a religious book and that he deleted the book "after realizing that its contents discussed jihad."