NEW YORK - Until a suspected terrorist plot was revealed, few people even knew there was a pipeline of highly combustible jet fuel snaking beneath the nation's largest city.

But authorities said Monday that it's one of countless lesser-known targets - from waterway retaining walls to dingy rail yards to tunnel ventilation systems - that they struggle to protect from attacks.

The New York Police Department “spends considerable time and resources protecting the landmarks nearly everyone would recognize as emblematic of New York and America,” said Police Commissioner Raymond Kelly.

“But we also protect the anonymous, unheralded elements of infrastructure that are essential to the life of the city.”

Police were aware that the fuel system feeding John F. Kennedy International Airport posed a risk well before investigators unearthed an alleged conspiracy by a homegrown Muslim cell to blow it up, with the far-fetched goal of killing thousands of people and inflicting major damage on the U.S. economy.

“It's been a concern for us for a long time,” said NYPD spokesman Paul Browne.

But pipeline networks don't carry the stature of places like Wall Street, the Empire State Building and the Statue of Liberty, so the general public doesn't give them much notice.

Through an exhaustive threat analysis, authorities have identified numerous other targets that, though often involving mundane infrastructure, pose dire risks.

Such targets “are pretty boring,” said Kelly McCann, president of Kroll Security Group and a former Marine officer assigned to counterterrorism duty.

“People tend not to think about them.”

But, McCann added, federal and local authorities “think about them every day. They're fully focused on them and feeling overwhelmed.”

For example, the NYPD has monitored the hulking ventilation towers along the Hudson River that feed fresh air to the Lincoln and Holland tunnels for signs of a chemical attack.

Tens of thousands of cars pass through the tunnels each day, and they are usually filled with bumper-to-bumper traffic during rush hours.

If a nefarious substance got in the tunnel, it could be devastating.

The NYPD has also quietly used its scuba unit to inspect retaining walls on the East River near the United Nations for any signs that someone may have planted underwater mines.

Officers have also sought to secure rail yards, fearing that terrorists might try to tamper with rail cars carrying dangerous chemicals through the city.

Past investigations have foiled a separate scheme to flood the Wall Street area by breaching a retaining wall at the World Trade Center site, along with a homegrown plot to blow up the busy Herald Square subway station in midtown Manhattan.

Investigators in the 2004 train bombing in Madrid discovered a crude diagram of Grand Central Terminal on a computer disk seized from one of the suspect's homes.

The pipeline system targeted by the suspects in the latest case is huge, winding its way through all corners of the city.

It is part of a national pipeline network run by Buckeye Partners LP that serves major airports in New York, Chicago, Detroit and Pennsylvania, among others.

The network also delivers jet fuel to military bases.

A criminal complaint outlining the case quotes one suspect as saying he hoped it would “cause greater destruction than in the Sept. 11 attacks,” torching the airport, killing several thousand people and destroying parts of New York's borough of Queens, where the pipeline runs underground.

On Monday, a law enforcement official confirmed reports that the suspects had been captured on tape discussing trying to recruit Adnan Gulshair El Shukrijumah, an alleged al-Qaida member and bombmaker.

The suspects allegedly sought support from fellow extremists in Trinidad and Guyana, where El Shukrijumah grew up.

But there was no evidence that the men ever made contact with El Shukrijumah, said the official, who spoke on condition of anonymity because the investigation had not been completed.

Law enforcers wouldn't have interrupted the terror plot if there was a chance their investigation could have led them to El Shukrijumah, the official said.

Authorities and experts agree that the scheme - like the recent plot to attack a military base in New Jersey - was another case of the suspects' ambitions far exceeding their capabilities.

The pipeline was designed to shut off when it detects heat, preventing a chain reaction explosion that the plotters allegedly envisioned, they said.

And it would have been virtually impossible to get inside the airport to do the kind of damage they planned.

Even if the suspects had succeeded in detonating a bomb, McCann said, there “would have been some damage and it would have been terrible. But would the whole thing have gone up? No.”