Few New Yorkers noticed earlier this summer when a dozen police horses boarded in a stable located in lower Manhattan for most of the 20th century were loaded into trailers and moved uptown.

The New York Police Department relocated the horses -- a quaint curiosity to neighbors living in high-end TriBeCa lofts and townhouses -- to build a temporary staging area for 220 officers newly assigned to protect Ground Zero.

The lower Manhattan force will eventually rise to 670 -- larger than any of the 76 precincts in the five boroughs and entire departments across the country.

The multiple thousands who will visit the Sept. 11 memorial after it opens this fall will endure airport-style screening and be watched by hundreds of closed-circuit cameras as part of the attack site opens publicly for the first time since 2001.

Securing the World Trade Center site from terror attacks has been one of law enforcement's most pressing problems long before the al-Qaida attack that destroyed the towers.

The resurrection of the 16-acre property may be viewed by most Americans as a triumph of the nation's resolve, but local law enforcement officials believe terrorists see it as another chance to prove their tenacity.

The site isn't the target of a current known plot, but it "remains squarely in the terrorists' crosshairs," said Police Commissioner Raymond Kelly.

Fears of a repeated plot against the site years ago moved its signature skyscraper several feet from its original spot.

An original plan putting 1 World Trade Center 25 feet off a state highway near the Hudson River raised concerns by the NYPD that it could be vulnerable to car or truck bombs. A redesign moved it farther off the street and incorporated a windowless 200-foot base.

To make the base of the 1,776-foot tower less bunker-like, the new plan called for a facade of 2,000 glass panels attached to aluminum screens. But tests showed that the glass failed to shatter into harmless bits as hoped and the Port Authority, which owns the site, had to send architects back to the drawing board.

Developers and law enforcement also have grappled with how to best police the anticipated steady flow of tourists, workers and commerce at the site without turning it into an inhospitable, armed camp.

Police plan to use a vehicle security center to screen tour buses, trucks and cars before they enter the site and park or makes deliveries using an underground roadway.

Pedestrian traffic, including visitors to the museum, also will be screened before they can enter the central plaza.

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