United States counterterrorism officials have advised nationwide police that terrorists could have interest in attacking the country's stadiums, entertainment complexes and hotels.
Police around the U.S. received bulletins from both the FBI and the Department of Homeland Security Monday, saying that officials knew of no specific plots, but urging law enforcement and security companies to stay vigilant for any threats.
The bulletins indicate that such sites are attractive targets for al Qaeda and other groups.
More specifically, the bulletins cite an al Qaeda training manual that says "blasting and destroying the places of amusement, immorality and sin...and attacking vital economic centres" is a desirable attack strategy.
A joint statement issued by Homeland Security and the FBI 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."
While the memos contain little concrete information, they do serve a two-fold purpose, according to one terrorism expert.
"The lesser aspect is it's just protection in case anything does happen, that they can say, 'Look, we warned everyone, we've executed our diligence as best as we can,' so that if something does occur, that they're not held culpable for hanging on to information that might have prevented an attack elsewhere," security expert John Thompson said Tuesday during an interview with Â鶹´«Ã½ Channel.
"The primary concern is that this is sort of an early notice to other police forces, to other police forces in other countries, and to other governments that this is another investigation that might, like so many others, be extremely wide-ranging."
The memos did not make mention of an ongoing investigation involving a Colorado man who was arrested in New York City on Saturday.
Officials believe Najibullah Zazi, 24, an Afghanistan-born immigrant who drives a shuttle van at the Denver airport, may have been plotting to set off bombs in backpacks on New York City trains, much like recent attacks on public-transit systems in Madrid in London.
Zazi and two others have been detained, though they are so far only charged with minor offences, including lying to authorities.
In the wake of the investigation, the New York transit authority said Tuesday it had increased the police presence around the city.
Officers donning helmets, bullet-proof vests and large firearms could be seen at busy city locations, such as Grand Central Terminal.
While exactly how many suspects there are in the investigation is unclear, New York City Police Commissioner Raymond Kelly said the probe is ongoing and that, "There's a lot more work to be done."
Last Friday, the FBI and the Department of Homeland Security warned that improvised explosive devices are the most common tactic used to target transit systems overseas.
That bulletin urged transit systems to conduct random sweeps at its stations and to have police make random patrols of trains and buses.
According to Thompson, more charges could be laid in the New York transit case as investigators unravel all of the details.
"In counter-terrorism, the whole strategy is pre-emption and prevention," he said. "So often when you start to pick up a cell, you haven't actually got them for any serious offences. In fact, all you've got before you even arrest them is just your suspicions because of who they are networking with and where they may have gone."
With files from The Associated Press
