Three people are in custody overseas for their suspected involvement in a plot to attack New York City's mass transit system, the FBI says.
Plotters are said to have talked of flooding New York's financial district
They are believed to be among "eight principal players" who planned the suicide bombing of a tunnel under the Hudson River, officials added.
A man accused of masterminding the plan has already been charged in Lebanon.
The FBI said the suspects were al-Qaeda followers who had been in the early stages of plotting their attacks.
"This is a plot that would have involved martyrdom, explosives and certain of the tubes that connect New Jersey with Lower Manhattan," Mark Mershon, assistant director-in-charge of the FBI New York Field Office, told a news conference.
"We're not discussing the modality beyond that."
Mr Mershon told reporters that, as well as the arrest in Lebanon, they had two others in custody "again in the foreign environment".
"We believe we have what I'll call eight principal players. And that we have them largely identified," he went on. "As I say, some are in custody, one of those has been charged formally in Lebanon."
The man charged in Lebanon has been named as Assem Hammoud, who reportedly used the alias Amir Andalousi, who was taken into custody in April.
Lebanese officials said he had confessed to the plot and being a member of al-Qaeda.
The plot is said to have been uncovered during routine monitoring of internet chatrooms used by extremist groups.
Mr Mershon conceded that the alleged cell had no explosives and had never been to the US but insisted it represented a serious threat.
Democratic Senator Charles Schumer of New York said this was one instance "where intelligence was on top of its game and discovered the plot when it was just in the talking phase".
A number of plots targeting subways, tunnels and other New York City landmarks have come to light since the 2001 attacks on the World Trade Center.