New York ? A terrorist plot to flood lower Manhattan by attacking train tunnels under the Hudson River used by tens of thousands of commuters was thwarted before the conspirators could travel to the United States, authorities said Friday.

Eight suspects – including an al-Qaida loyalist arrested in Lebanon and two others in custody elsewhere – had hoped to pull off the attack in October or November, federal officials said. But federal investigators working with their counterparts in six other countries intervened. The other five suspects remained at large.

“It was never a concern that this would actually be executed,” Homeland Security Secretary Michael Chertoff said in Boston. “We were, as I say, all over this.”

Initial reports said the terrorists wanted to attack the Holland Tunnel, a major thoroughfare for cars entering the island of Manhattan. But officials said the group had specifically mentioned only the train tunnels under the Hudson River used by commuters.

Police officers stand by as New Jersey-bound traffic streams into the New York entrance of the Holland Tunnel in New York. Law enforcement officials announced Friday that they have disrupted planning by foreign terrorists for an attack on New York City tunnels under the Hudson River.

“This is a plot that involved martyrdom and explosives” and focused on the “tubes that connect Jersey and lower Manhattan,” said FBI Assistant Director Mark J. Mershon.

The men thought that by bombing the train tunnels they could unleash massive flooding in lower Manhattan, home to Wall Street and the World Trade Center site, New York Police Department Commissioner Ray Kelly said.

Investigators decided in recent weeks that the “plotting for this attack had matured to a point where it appeared that the individuals were about to move forward,” Mershon said. “They were about to go to a phase where they would attempt to surveil targets, establish a regimen of attack and acquire the resources necessary to effectuate the attacks.”

Officials cited the arrest of the Lebanese suspect – described as the scheme’s mastermind – as a significant break in the investigation. A Lebanese official said the Beirut man confessed to plotting to attack New York City tunnels later this year, and that he was acting on Osama bin Laden’s orders.

Police arrested the man April 27, acting on information from the FBI. The 31-year-old suspect uses the alias Amir Andalousli, but his real name is Assem Hammoud.