BEIJING, July 10 (Xinhuanet) -- A web of public and private security cameras is being created in lower Manhattan to keep an eye on troublemakers, the New York Police Department announced Monday.

Yet, there are questions about whether such surveillance devices indeed serve their purpose.

By the end of this year, 116 license plate readers will monitor cars moving through the city's financial district. "This area is very critical to the economic lifeblood of this nation," New York City's police commissioner, Raymond W. Kelly, said in an interview last week. "We want to make it less vulnerable."

The surveillance will eventually include 3,000 public and private security cameras to track terrorists.

The cameras have provided valuable forensic evidence in crime and terror investigations, such as the recent attempted car bombings and the July 7, 2005, terror attacks in London where British officials were able to track the movements of the perpetrators and make arrests.

But critics ask whether such surveillance devices indeed serve their purpose. These cameras are considered to be effective at documenting crime, but less effective at reducing it, they argued.

In England, the use of cameras exploded after the 1993 Bulger case in which two 11-year-old boys were videotaped kidnapping 2-year-old Jamie Bulger at a shopping center in Liverpool.

Though the cameras did not prevent Bulger's murder, the evidence they provided did help convict the killers, the police said.

"They are good forensic tools — after something happens, they'll tell you what happened," said Jim Harper, the director of information policy studies at the Cato Institute. "And in the rare case where a terrorism case fails, they can be useful to help track down the perpetrators. But they do not provide protection against attacks, and that's a key distinction."

(Agencies)