DENVER (AP) - An airport shuttle driver under arrest in Colorado 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, officials said.
The investigation into the possible terror plot has prompted counterterrorism officials to warn mass-transit systems around the nation to step up patrols.
Two law enforcement officials speaking on condition of anonymity because they were not authorized to discuss details of the investigation told The Associated Press late Monday that more than a half-dozen individuals 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 remained 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.
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 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.
The effects of the warning were not immediately clear Monday. New York's transit agency said it was in touch with an FBI-NYPD task force but wouldn't comment further.
The task force 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.
Investigators 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-Qaida 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."
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 the 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 place 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.
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," 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."
Afzali 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.
His attorney, Ron Kuby, accused authorities of trying to make Afzali a scapegoat for a botched investigation. Kuby told reporters outside court that before Afzali's arrest, authorities had begged him to help them in the Zazi investigation. He said his client knew he was being recorded, and never tried to mislead the FBI.
"They blew their own investigation and now they're trying to blame my client," he said.
Zazi's father could be released Thursday and placed under electronic monitoring at home and have his passport confiscated.
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.