Updated 10:15 a.m. Eastern:
Counterterrorism officials warned mass transit systems, sports stadiums and hotels around the nation Monday to step up security because of fears an Afghanistan-born immigrant under arrest in Colorado may have been plotting with others to detonate backpack bombs aboard New York City trains.
Investigators say Najibullah Zazi, a 24-year-old shuttle van driver at the Denver airport, played a direct role in a terror plot that unraveled during a trip to New York City around the anniversary of the Sept. 11 attacks. He made his first court appearance Monday and was ordered to remain in custody pending a detention hearing on Thursday.
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.
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.
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.
CBS News learned Monday that despite the arrests, law enforcement officials still have "deep concern" that the government may have .
Investigators said they found notes on bomb-making instructions that appear to match Zazi's handwriting on his laptop, and discovered his fingerprints on materials - batteries and a scale - that could be used to make explosives.
Orr reports that a cell phone found in Zazi's possession contained a video of New York's Grand Central Station, which, combined with a reported admission that he intended to carry out bomb attacks, prompted the warning to mass transit systems.
Counterterrorism officials also have issued security bulletins about terrorist interest in attacking sports stadiums, entertainment complexes and hotels.
However, Orr says the warnings, while legitimate, should not envoke panic - they are routine advisories, and the government puts out numerous such alerts every month.
The FBI still has no hard, specific evidence pointing to any immenent plot against America's transit systems or other targets, reports Orr.
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.
According to a complaint filed in the case, Zazi "received instruction from al Qaeda operatives on … weapons and explosives."
Publicly, law enforcement officials have repeatedly said they are unaware of a specific time or target for any attacks. However, speaking on condition of anonymity because they were not authorized to discuss the case, officials say investigators have worried most about the possible use of backpack bombs on New York City trains, similar to attacks carried out in London and Madrid.
The investigation into Zazi's role and how many others may be involved was ongoing. Two law enforcement officials speaking on condition of anonymity told The Associated Press late Monday that more than a half-dozen individuals were being scrutinized in the alleged plot.
The FBI said in a statement that "several individuals in the United States, Pakistan and elsewhere" were being investigated.
Backpacks and cell phones were seized last week from apartments in Queens where Zazi visited.
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. They noted incidents in which bombs were made with peroxide.
In the bulletin, obtained by The Associated Press, 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.
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."
A strange sequence of events began to unfold nearly two weeks ago when Zazi - already under surveillance by federal agents - rented a car in Colorado and made a 1,600-mile trek across the heartland to New York. He told reporters that he went to New York to resolve an issue with a coffee cart he owned.
He was briefly stopped entering the city as part of what was believed to be a routine drug check, and proceeded to his friend's home in Queens. Once there, his car was towed and authorities confiscated his computer. He was told by an NYPD informant that detectives were asking about him, and decided to cut the trip short and fly back to Colorado, authorities said.
Their surveillance blown and their main suspect flying back to Colorado, officials speeded up the investigation and launched raids on several Queens apartments in a search for evidence of explosives.
"Whatever investigative interest this guy held prior to that time, when it became clear he was leaving for New York shortly before Sept. 11, my guess is he became a much brighter blip on their radar screen," said Pat Rowan, the former head of the Justice Department's National Security Division.
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.
On Monday, Najibullah Zazi answered the judge's questions politely with a "Yes, honor" or "No, honor."
Arrest Affadavit: Najibullah Zazi
Arrest Affadavit: Mohammed Wali Zazi
Arrest Affadavit: Ahmad Wais Afzali
Afzali, with a long dark beard and wearing a tunic, was ordered held without bail after prosecutors said they believed he might flee if released. He smiled and blew kisses to his wife and other relatives before deputy marshals led him out of the courtroom.
All face the same charge of lying to the government in a matter involving terrorism. If convicted, they face eight years in prison.
Mohammed Zazi was appointed a federal public defender, Warren Williamson, and was expected to be released within 48 hours.
Father and son made eye contact several times in court Monday but didn't speak to one another.
U.S. prosecutor Tim Neff said the proposed terms of release include a $50,000 unsecured bond, meaning he wouldn't have to pay unless he broke bond, along with electronic monitoring at his home and a ban on leaving Colorado. Authorities want to take away his U.S. passport, Neff said.
Mohammed Zazi and Afzali are accused of lying to FBI agents about calls between Denver and New York. An affidavit accuses Afzali of lying about a call in which he told Najibullah Zazi that he had spoken with authorities.
Zazi's father is accused of lying when he told authorities he didn't know anyone by the name of Afzali. The FBI said it recorded a conversation between Mohammed Zazi and Afzali.
"The parts don't fit together," said CBS News legal analyst Andrew Cohen. "The whole story clearly hasn't been told. If the facts in the supporting documents are true and could be proven, you would think the Zazis and their New York counterpart would already have been charged with providing material support to a terror group - and yet that hasn't happened.
"One theory is that the feds hoped (or still hope) that these three men will help them get information about other potential suspects. The problem with that theory is that if these men didn't do anything more than lie to authorities, there may not much of a larger story for them to tell."
Neff said the judge wanted time to review any secret information collected by the FBI under the Foreign Intelligence Surveillance Act before determining how to proceed.
Afzali appeared in federal court Monday in Brooklyn and was ordered held without bail. His attorney, Ron Kuby, said he would seek bail Thursday.
Zazi was born in Afghanistan, moved to Pakistan at age 7 and emigrated to the United States in 1999. He returned to Pakistan in 2007 and 2008 to visit his wife, according to Folsom.
Since 2001, counterterrorism officials have shifted their approach and made the disruption of plots in their early stages a top priority, ahead of amassing incriminating evidence of more serious crimes. The exceptions to the rule are plots infiltrated by informants who are being directed by the FBI every step of the way.
"In the current environment when plotters are disrupted before their plot becomes concrete, you may end up with something that looks relatively trivial to the legal system, but the truth is you can't judge their efforts by the legal charges they're able to bring," Rowan said.
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