[Batpuff]

NEW YORK — The suspects in a terrorist plot targeting Lower Manhattan hoped to unleash a catastrophic flood by destroying a huge underground wall that keeps Hudson River water out of the World Trade Center site, two law enforcement officials said Thursday.

It was unclear how the suspects hoped to bring down the so-called slurry wall, which was quietly put under 24-hour police protection in recent weeks once details of the plot began to emerge, the officials told The Associated Press, speaking on condition of anonymity because the scheme was still under investigation.

A yearlong investigation had revealed that the eight suspects "specifically wanted to take out the slurry wall in hopes of flooding the financial district," one of the officials said.

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The vulnerability of the wall — viewed each day by hundreds of tourists visiting ground zero — became a concern for law enforcement and engineers following the Sept. 11 terror attacks.

The FBI and police did not mention the retaining wall as a target last week while confirming reports that plotters envisioned an attack in the fall on rapid transit tunnels, which run under the river and connect to the Trade Center site. The plot also allegedly involved suicide bombers on trains

The FBI and NYPD declined comment Thursday.

Officials last week said FBI agents monitoring Internet chat rooms used by overseas extremists learned of the plot. They described the conversations as mostly extremists discussing and conceptualizing the plot, which was disrupted before suspects could come to New York and become a more serious threat.

Officials cited the arrest of a Lebanese suspect from Beirut — described as the scheme's mastermind — as a significant break in the investigation. Lebanese authorities said Assem Hammoud claimed he was acting on Usama bin Laden's orders.

The slurry wall was built to surround the foundations of the twin towers when they were being built in the late 1960s, resisting pressure from the Hudson River and waterfront land. The wall is 3-feet thick, more than 70-feet deep and stretches about four blocks.

The towers also helped hold the river back; when they were destroyed — leaving a 16-acre, 70-foot deep construction site — there was little to support the wall, except for some protruding steel cables.

Engineers have worked over the years to reinforce the wall while the site is being rebuilt. The estimated cost of repairing it contributed to the once $1 billion proposed price tag for the planned Sept. 11 memorial.

The site's owner, the Port Authority of New York and New Jersey, has since agreed to pay $150 million to shore up the wall and do other work to make sure the memorial can be built on the spots where the towers stood