Surveillance cameras like those authorized by the D.C. Council for police investigations and now being put in place have shown limited success in decreasing violent crime in other cities.
Baltimore, for example, set up about 80 cameras in May 2005 in high-crime neighborhoods. Volunteers and retired law-enforcement personnel monitor the images in real time, but the cameras have not helped put criminals behind bars.
"Generally, the State's Attorney's Office has not found them to be a useful tool to prosecutors," office spokeswoman Margaret Burns said. "They're good for circumstantial evidence, but it definitely isn't evidence we find useful to convict somebody of a crime."
Miss Burns said Baltimore prosecutors kept detailed statistics from the first nine months of the camera program. Most of the 500 cases forwarded to prosecutors were quality-of-life crimes, she said, and 40 percent of those cases were dropped by prosecutors or dismissed by the courts.
"We have not used any footage to resolve a violent-crime case," she said.
Miss Burns said police sometimes misidentify suspects because the cameras produce "grainy" and "blurry" images.
"We have had that happen more than once," she said.
The D.C. Council, faced with a sharp increase in crime, passed emergency legislation July 19 that allows the Metropolitan Police Department to use surveillance cameras in neighborhoods as part of an emergency plan.
D.C. workers on Thursday began installing the first four of an expected 47 cameras throughout the city. Officials said the four cameras are temporary and will be replaced by permanent ones later this month. About 24 cameras will be deployed by the end of August, and 23 more will be added in September, police said.
Police Chief Charles H. Ramsey is required to notify only two persons about plans to place a camera in any given neighborhood: an advisory neighborhood commissioner and the appropriate council member. The cameras will operate 24 hours a day, but police will review the images only when a known crime may have been recorded.
Chicago deployed a few dozen cameras in neighborhoods in July 2003. Authorities there captured their first drug transaction 19 months later, in February 2005.
Police arrested three suspects and confiscated 12 packets of heroin. However, the cameras have not helped in criminal investigations.
"From my perspective, I would love it if we had footage of the murderer leaving the house, but that hasn't happened yet," said Kevin Smith, a spokesman for Chicago's Office of Emergency Management and Communications, which administers and monitors the 170-camera network.
Police in San Francisco said a camera paid off in an investigation for the first time in June, when they arrested a man in connection with a shooting in April.
Nine months after the first cameras were installed in neighborhoods, a camera captured the image of a man getting out of a car. The man subsequently shot at another man and missed, injuring a 13-year-old girl. The image was not recorded, but police said the camera was key to the investigation.
Surveillance cameras also have generated headlines for the wrong reasons.
In April 2005, a San Francisco police officer was suspended from the department for using surveillance cameras to ogle women at San Francisco International Airport.
New York officials say surveillance cameras in public-housing projects have led to substantial decreases in crime.
Written policies and random audits help guard the system against abuse, but that proved ineffective when the tape of a 22-year-old man who fatally shot himself in the lobby of a housing project in March 2004 surfaced on a pornographic Web site.
Critics argue that cameras only push criminals into unobserved areas. A University of Cincinnati study in 2000 concluded that surveillance cameras have a short-term deterrent effect, which likely would increase when the public is notified about their presence.
Cameras in Baltimore, Chicago, New York and San Francisco are labeled as police property. No police department logos are affixed to the D.C. cameras that were in place before the recently crime emergency.
D.C. police spokesman Kevin Morison said police are required to post signs indicating that an area is under surveillance. He could not say whether such notification would be required under a clause dealing with "exigent" circumstances.
Mr. Morison said several neighborhood leaders have requested cameras.
Marc Rotenberg, executive director of the District-based Electronic Privacy Information Center, said he has heard neighborhood leaders express approval of the cameras at hearings but is not sure whether most residents share that support.
"It's very difficult to get a clear read on whether this is something that residents really want," Mr. Rotenberg said. "I don't think people understand that if you put these cameras in residential communities, you're talking about a telescopic lens that can zoom in and a 360-degree casing that can look into your bedroom."