NEW YORK - The New York Police Department is working on a plan to track every car, truck or other vehicle entering Manhattan and screen them for radioactive materials and other terrorism threats.

The ambitious proposal, called Operation Sentinel, is being developed alongside a separate $90 million security initiative to tighten security at the World Trade Center site and elsewhere in lower Manhattan in response to the attacks of Sept. 11, 2001.

Police officials say Operation Sentinel would rely on license-plate readers, radiation detectors and closed-circuit cameras installed at the 16 bridges and four tunnels serving Manhattan. About a million vehicles drive onto the island every day.

The vehicle data - license plate numbers, radiological readings and photos - would be automatically analyzed by computers programmed with information about suspicious vehicles.

Police say the system could help them intercept would-be attackers before they can do harm.

There is no estimate yet of the cost, since Operation Sentinel is in just the planning phase.

The proposal already has raised red flags among civil rights advocates.

"We think that Operation Sentinel and a lot of the surveillance initiatives that the police are planning are an attack on our right to privacy here in New York," said Matt Faiella, a staff attorney for the New York Civil Liberties Union.

Faiella fears innocent motorists could end up in "a database of all of their movements and faces."

Police say that law-abiding people have nothing to fear: Vehicle data deemed innocent would be purged from police records after 30 days.

Investigators need "time to maintain this information to enable us to check and see if any information has surfaced concerning a vehicle coming into the city," Police Commissioner Raymond Kelly said. "We don't think people should have cause for concern."

The proposal was contained in a security plan prepared by the NYPD's counterterrorism division that also outlined measures already being implemented to secure lower Manhattan in the post-Sept. 11 world.

That plan calls for 116 stationary and mobile license-plate readers and 3,000 closed-circuit cameras that would be monitored by officers at a command center. It was modeled in part after the "ring of steel" surveillance measures in London's financial district.

"We have worked closely with London authorities," the commissioner said. "But I would submit this will be a much more advanced system than they have in place."

The NYPD's auto theft division already uses unmarked police vehicles with two roof-mounted cameras to read license plates of passing and parked cars. Computers in the police vehicles compare the scanned plate numbers with a database of stolen and suspicious cars. That system has assisted in the recovery of hundreds of stolen cars and scores of arrests, police said.

© Copyright 2008 Globe Newspaper Company.