A wall that suspects in a terrorist plot hoped to destroy to unleash a catastrophic flood in lower Manhattan was quietly put under 24-hour protection in recent weeks once details of the plot began to emerge, two law enforcement officials said.

The suspects hoped to bring down the so-called slurry wall, which keeps the water from the Hudson River out of the World Trade Center site, the officials told The Associated Press Thursday. They spoke on condition of anonymity because the scheme was still under investigation.

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A yearlong investigation 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.

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.

"You want to make sure that that gets a little extra protection because it is, in theory, more likely to be a target," New York Mayor Michael Bloomberg said of the trade center site. "The 'bathtub' itself normally would be held back by all of the steel structure underneath; that steel structure isn't there at the moment."

"We just want to make sure that nobody thinks that they could cause more chaos down there," Bloomberg said on his weekly show on WABC Radio.

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.

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The FBI and New York Police Department declined to comment.

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 — described as the scheme's mastermind — as a significant break in the investigation. Lebanese authorities said the Beirut man 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 to 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 it while the site is being rebuilt.

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