The plot had eight principal players, three in custody, including the mastermind, 31-year-old Lebanese national Assem Hammoud, the only one of the suspects currently charged in his home country.
"This is a plot that involved martyrdom and explosives and certain of the tubes that connect Jersey and lower Manhattan," FBI Assistant Director Mark J. Mershon said. He called the plot the "real deal."
"We believe we intercepted this group early in their plotting and, in fact, the plan has largely been disrupted," Mershon said.
Initial reports said the terrorists wanted to attack the Holland Tunnel. But Mershon said the group specifically mentioned only the PATH train tunnels between New York and New Jersey.
A Lebanese security official said Hammoud, a Beruit native, confessed to plotting to attack New York City tunnels in October or November of this year and said he was acting on Osama bin Laden's orders. The official spoke on condition of anonymity because the investigation is ongoing.
Hammoud told investigators he was acting "on a religious order from bin Laden and said 'I am proud to carry out his orders'," the official said.
A Lebanese Interior Ministry statement said Hammoud was arrested April 27 and that he possessed detailed maps of the targets, CBS News reports.
Hammoud planned to go to Pakistan for four months for training and had already undergone some light weapons training with a Syrian man who came to Lebanon this year. He had communicated on an "extremist militant" website and was under instructions to act non-religious. "He did just that with perfection," the statement said.
New York's transportation system has emerged as a potential terrorist target in several recent cases. A June book by journalist Ron Suskind highlighted a reported plot by al Qaeda terrorists to kill thousands of New Yorkers by spreading cyanide gas in the subway. In May, a man was convicted of plotting to blow up a subway station.
In the latest case, a federal official said FBI agents monitoring Internet chat rooms used by extremists learned of the plot in recent months and determined that tunnels were possibly being targeted after investigators pieced together code words from their conversations.
The probe began one year ago as a result of FBI intelligence gathering, with six foreign governments assisting.
"At this time we have no indication of any imminent threat to the New York transportation system, or anywhere else in the U.S.," Richard Kolko, Washington-based FBI special agent, said in a statement to Associated Press Radio.
One U.S. official called the plot "largely aspirational" and described the Internet conversations as mostly extremists discussing and conceptualizing the plot. The official said no money had been transferred, nor had other similar operational steps been taken.
CBS News correspondent Jim Stewart reports his sources say that no one in the United States ever took part in the Internet conversations and that no one ever purchased any explosives or scouted the transit system.
"It was never a concern that this would actually be executed," Homeland Security chief Michael Chertoff said Friday. "We were, as I say, all over this."
Details of the plot emerged on the one-year anniversary of the attacks on the London transportation system that killed 52 people.
"This is one instance where intelligence was on top of its game and discovered the plot when it was just in the talking phase," said Sen. Charles Schumer, D-N.Y.
A U.S. counterterrorism official said Friday that investigators had found no evidence that the Holland Tunnel was part of the plot. The New York Daily News had reported Friday that the plotters wanted to blow up the tunnel, the southernmost link between Manhattan and New Jersey, in the hopes of flooding New York's financial district.
In its statement, Homeland Security and the FBI said, the investigation was ongoing.
"We know al Qaeda continues to have an interest in attacking the United States," it added. "At this point in time, there is no specific or credible information that al Qaeda is planning an attack on U.S. soil."