Federal counterterrorism officials warned local police to patrol stadiums, hotels and entertainment complexes for suspicious activity after the arrest of a Colorado shuttle driver and others suspected in a potentially far-reaching terror plot.
But officials said the FBI/Homeland Security bulletins weren't tied to known threats and police in Washington D.C. and New York City downplayed terror fears.
In the nation's capital, intelligence and transit authorities said there isn't a threat to the D.C. Metro system and there has been "no additional elevation of security following the recent arrest of a terror suspect in Colorado."
"We have no specific information from intelligence sources that would lead us to believe there is a threat to our system at this time, yet MTPD (Metro Transit Police Department) officers remain vigilant," said Metro Police Chief Michael A. Taborn in a news release.
New York City Police Commissioner Ray Kelly also said there wasn't a specific threat in his city and he had no plans to step up his forces as a result.
"There is no security information on entertainment, sports, transit, no specific information ... other than the fact that in a post-9/11 world, we are concerned about a lot of things," Kelly told reporters at a Tuesday morning briefing in Manhattan.
Kelly said New York City already devotes "1,000 or over 1,000" police officers daily to counterterrorism efforts.
"We have not engaged in any different practices or increased our security," he said. "We are, every day, up to a level that exceeds any other city that I’m aware of."
New York City's transit agency said Tuesday it has increased police presence at "key locations" — including Grand Central Terminal — in light of a continuing terrorism probe. But the Metropolitan Transportation Authority (MTA) added there was no credible threat to the city's subway system and commuter trains.
In Washington, a handful of police cars were stationed Tuesday outside The Verizon Center, where the Wizards and Capitals play basketball and hockey.
The FBI and Department of Homeland Security alerts also cautioned police around the country to watch storage centers for any unusual behavior.
Two bulletins were sent Monday to local authorities nationwide, after a similar bulletin about mass transit systems was sent out. The warnings — the latest in a flurry circulated as authorities investigate a possible bomb plot in Denver and New York City — say the sites mentioned remain attractive targets to groups like Al Qaeda.
Three people are charged with lying to officials investigating the alleged plot.
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.
The advice to watch storage unit was given because the London bomb plot was carried out by terrorists who hid their materials in small public storage spaces, a source told FOX.
Local law enforcement are told to look for "suspicious behavior with regard to storage facilities" where people can rent space to keep their belongings, the source said. Authorities are trying to determine whether there are any more explosive-making substances that still haven't been tracked down.
The FBI and DHS 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."
Law enforcement bulletins like the FBI/DHS ones obtained by the press are not intended for the public. Bulletins — particularly about hotels as possible targets — are common, and often don't make news. But a half-dozen alerts issued in the last week have received increased attention amid the ongoing New York-Denver investigations.
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.
The investigation and the earlier warning about mass transit system have already prompted officials around the nation to step up patrols.
Two law enforcement officials speaking on condition of anonymity told The Associated Press late Monday that more than a half-dozen people were being scrutinized in the alleged plot.
In a statement, the FBI says that "several individuals in the United States, Pakistan and elsewhere" are being investigated.
Investigators say Najibullah 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 attacks.
He made his first court appearance Monday and remains behind bars.
Zazi and two other defendants have not been charged with any terrorism counts, only the more minor offense of lying to the government. More charges could come later.
Backpacks and cell phones were seized last week from apartments in Queens, where Zazi visited.
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.
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, similar to attacks carried out in London in 2005 and Madrid in 2004.
Backpack bombs ripped apart four commuter trains and killed 191 people in Madrid on March 11, 2004. On July 7 the next year, bombing attacks in London killed 52 subway and bus commuters.
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, 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."
Zazi and his 53-year-old father, Mohammed Wali Zazi, were arrested Saturday in Denver. Ahmad Wais Afzali, 37, was arrested in New York, where he is an imam at a mosque in Queens. The three are accused of making false statements to the government. If convicted, they face eight years in prison.
FOX News' Catherine Herridge, Mike Levine, David Lee Miller and The Associated Press contributed to this report.