Counterterrorism officials have issuedsecurity bulletins to police around the nation about terrorists'desire to attack stadiums, entertainment complexes and hotels - thelatest in a flurry of such internal warnings as investigators chasea possible bomb plot in Denver and New York.

In the two bulletins - sent to police departments Monday andobtained by The Associated Press - officials said they know of nospecific plots against such sites, but urged law enforcement andprivate companies to be vigilant. These two bulletins followed onthe heels of a similar warning about the vulnerabilities of masstransit systems.

The bulletin on stadiums notes that an al-Qaida training manualspecifically lists "blasting and destroying the places ofamusement, immorality, and sin... and attacking vital economiccenters" as desired targets of the global terror network.

A joint statement from DHS and FBI said while the agencies"have no information regarding the timing, location or target ofany planned attack, we believe it is prudent to raise the securityawareness of our local law enforcement partners regarding thetargets and tactics of previous terrorist activity."

Officials noted the law enforcement bulletins are not intendedfor the public. Bulletins - particularly about hotels as possibletargets - are common, and often don't make news. However, ahalf-dozen alerts issued in the last week have received increasedattention amid the ongoing 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 mayhave been planning with others to detonate backpack bombs on NewYork City trains in a terrorism plot similar to past attacks onLondon's and Madrid's mass-transit systems.

The investigation and the earlier warning about mass transitsystem have already prompted officials around the nation to step uppatrols.

Two law enforcement officials speaking on condition of anonymitybecause they were not authorized to discuss details of theinvestigation told The Associated Press late Monday that more thana half-dozen individuals were being scrutinized in the allegedplot.

In a statement, the FBI says that "several individuals in theUnited States, Pakistan and elsewhere" are being investigated.

Investigators say Najibullah Zazi, a 24-year-oldAfghanistan-born immigrant who is a shuttle van driver at theDenver airport, played a direct role in the terror plot thatunraveled after an overnight 1,600-mile trip from Denver to NewYork City around the anniversary of the Sept. 11 attacks. He madehis first court appearance Monday and remains behind bars.

Zazi and two other defendants have not been charged with anyterrorism counts, only the relatively minor offense of lying to thegovernment. But the case could grow to include more serious chargesas the investigation proceeds.

Backpacks and cell phones were seized last week from apartmentsin Queens, where Zazi visited.

Zazi has publicly denied being involved in a terror plot, anddefense lawyer Arthur Folsom dismissed as "rumor" any notion thathis client played a crucial role.

Publicly, law enforcement officials have repeatedly said theyare unaware of a specific time or target for any attacks.

Privately, officials speaking on condition of anonymity becausethey were not authorized to discuss the case said investigatorshave worried most about the possible use of backpack bombs on NewYork City trains, similar to attacks carried out in London in 2005and Madrid in 2004.

Backpack bombs ripped apart four commuter trains and killed 191people 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 SecurityDepartment warned that improvised explosive devices are the mostcommon tactic to blow up railroads and other mass transit systemsoverseas. And they noted incidents in which bombs were made withperoxide.

In that bulletin, obtained by The AP, officials recommended thattransit systems conduct random sweeps at terminals and stations andthat law enforcement make random patrols and board some trains andbuses.

Investigators feared Zazi may have been involved in a potentialplot involving hydrogen peroxide-based explosives, according to twolaw enforcement officials, who spoke on condition of anonymitybecause they were not authorized to discuss the investigation.

The FBI said they found notes on bomb-making instructions thatappear to match Zazi's handwriting, and discovered his fingerprintson materials - batteries and a scale - that could be used to makeexplosives. He also made a trip to Pakistan last year in which hereceived al-Qaida explosives and weapons training, the governmentsaid.

Zazi, a legal resident of the U.S. who immigrated in 1999, toldthe FBI that he must have unintentionally downloaded the notes onbomb-making as part of a religious book and that he deleted thebook "after realizing that its contents discussed jihad."

A strange sequence of events began to unfold nearly two weeksago when Zazi - already under surveillance by federal agents -rented a car in Colorado and made the 1,600-mile trek across theheartland to New York. He told reporters that he went to New Yorkto resolve an issue with a coffee cart he owned.

He went to his friend's place in Queens. Once there, his car wastowed and authorities confiscated his computer. He was told by anNYPD informant that detectives were asking about him, and decidedto cut the trip short and fly back to Colorado, authorities said.

Their surveillance blown and their main suspect flying back toColorado, officials speeded up the investigation and launched raidson several Queens apartments in a search for explosives, but foundnone.

Zazi and his 53-year-old father, Mohammed Wali Zazi, werearrested Saturday in Denver. Ahmad Wais Afzali, 37, was arrested inNew York, where he is an imam at a mosque in Queens. The three areaccused of making false statements to the government. If convicted,they face eight years in prison.