Authorities overseas have arrested one man and taken two others into custody on suspicion of planning suicide bombings in train tunnels beneath the Hudson River between Manhattan and New Jersey, officials said Friday. Five other men are being sought for their involvement in the plan, which law enforcement authorities said presented a genuine threat even though it was in its earliest stages and no attack was imminent.

The FBI and New York City police officialshave been aware of the group and its discussions for about a year, said Mark J. Mershon, the special agent in charge of the agency's New York office. Police presence at the tunnels in Manhattan that could have been targets has been increased in recent weeks in response to the investigation.

"The planning or the plotting for this attack had matured to the point where it appeared 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, and at that point I think it's entirely appropriate to take it down."

Federal and local law enforcement authorities identified the main subject of the investigation as Assem Hammoud, 31, a Lebanese man who was arrested on April 27 in Beirut and was still being held there. The locations of the other two men in custody were not revealed. The eight "principal players" planning the attack, the authorities said, had secured no financing, had gathered no explosives and had not visited New York — or even the United States — to conduct surveillance. At least one of the planners has been in Canada, the authorities said.

Monitoring of Internet chat rooms used by Islamic extremists led to the arrest of Hammoud, according to Lebanese authorities. At least one American official said the members of the group had never even met each other face to face.

"There was a lot of discussion, there was planning being done, but there was no indication that there was any movement toward these facilities," Police Commissioner Raymond W. Kelly said Friday. "There is no indication that materials were secured or that specific reconnaissance was done."

One counterterrorism expert who had been briefed on the plan, and who spoke on the condition of anonymity because he is not authorized to talk to the news media, said: "These are bad guys in Canada and a bad guy in Lebanon talking, but it never advanced beyond that. This is the talking phase."

He added: "They never were in New York, they never were in the States, they never got materials together. So in that regard, it's less serious than some of the others."

Mershon said an attack was to have been carried out in October or November; Lebanese authorities confirmed that timing. Mershon said Hammoud told Lebanese interrogators that he had pledged "allegiance to Osama bin Laden and he proclaims himself to be a member of al-Qaida." But it was not clear the suspect had ever interacted with bin Laden or his top deputies.

In a statement, the Lebanese Internal Security Directorate said that under questioning Hammoud had said he was a member of an extremist organization and had been planning a major bombing in the United States.

Separately, a senior security official in Beirut said Hammoud is thought to be the leader of al-Qaida in Lebanon, and that his arrest in April came under an Interpol order. Lebanese authorities wanted to publicize his arrest at the time, the security official said, but American authorities sought to keep the issue quiet, believing the investigation would lead to other information, the officials said.

Officials in Lebanon said he is from a religious family and lives with his mother in a building his family owns and teaches at a private university.

The arrest and the bombing plan were first reported Friday in The Daily News. It reported that the would-be suicide bombers had intended to blow a hole in the wall of the Holland Tunnel, allowing the Hudson River to flood the tunnel and Lower Manhattan.

But authorities said the focus appeared to have been on two PATH railroad tunnels, for the commuter trains between Manhattan and New Jersey. Those tunnels exit Manhattan at the World Trade Center and just south of Christopher Street.

Hammoud told his interrogators that one of the scenarios discussed was to put suicide bombers with explosives in backpacks on a PATH train to destroy the tunnel, said a law enforcement official who was granted anonymity because the case is ongoing. Another official said the number of planned bombers was seven or eight.

The first official said, "There was discussion about where to do it, how to do it, what it would take, what effect it would have in different gradations, that a key player was getting ready to depart to a country where we know was an al-Qaida presence." He noted, however, that Hammoud was not known to be "a major al-Qaida player."

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On Friday night, Kelly said during an appearance on "The NewsHour With Jim Lehrer" that the attackers "looked to, in some way, shape or form, open up the water — walls holding the water back — that would then go into the PATH tunnels that go under the Hudson River." Kelly suggested the plan involved the two tunnels and the PATH station near Pennsylvania Station. He said that way the water would enter the subway system as well.

Rep. Peter T. King, R-N.Y., the chairman of the House Committee on Homeland Security, said he had been briefed on the investigation for nine months. He said his understanding was that the target was not the Holland or Lincoln tunnels, which carry cars and trucks, but one of the two PATH tunnels.

Six foreign governments are assisting in the investigation, federal officials said, though they declined to identify them. Of the five suspects still being sought, Kelly said, "Their whereabouts are known, and they're being observed."