A pedestrian who strolls through Boston's Financial District, an area of about 40 city blocks, can be seen by at least 233 private and public cameras.
In the aftermath of the terrorist bombing there on April 15, Boston Police Commissioner Edward Davis wants even more cameras to boost street-level surveillance, said spokeswoman Cheryl Fiandaca. Other cities also now may be spurred to expand their systems, which security specialists said will fuel sales growth in the $3.2 billion video surveillance industry.
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Such actions increase tensions between law enforcement officials and privacy advocates, who say they worry about Big Brother intrusions into people's legal activities. The American Civil Liberties Union in San Francisco raised such concerns after Police Chief Greg Suhr cited Boston last week in saying he wants additional cameras for downtown Market Street to give police a better look during parades and other public events.
"We shouldn't rush into mass surveillance of San Franciscans as they go about their everyday lives," Abdi Soltani, executive director of the ACLU of Northern California, said on the group's website.
The role of video surveillance drew national attention as the FBI used law enforcement and private security cameras -- plus smartphone images provided by hundreds of people -- to identify the suspected bombers, brothers Tamerlan and Dzhokhar Tsarnaev. Since the al-Qaida terrorist attacks on New York and the Pentagon in 2001, U.S. cities have deployed video and other sensors purchased with the help of billions of dollars in federal counterterrorism funding.
"The Boston bombing is a terrible reminder of why we've made these investments -- including camera technology that could help us deter an attack, or investigate and apprehend those involved," New York City Mayor Michael Bloomberg said in remarks April 16.
The extent of government video surveillance capabilities in U.S. cities currently is hard to determine because the information is generally not made public.
Chicago authorities have access to about 10,000 public and private video surveillance cameras, according to a 2011 report by the American Civil Liberties Union of Illinois. San Francisco has cameras in high-crime areas that are reviewed for evidence after a crime has occurred.
In the U.S., closed-circuit television and video surveillance equipment are part of an estimated $3.2 billion market this year, up from $3 billion last year, according to IMS Research, part of IHS Inc. of Englewood, Colo.
The market is projected to grow to $4.1 billion by 2016, Paul Everett, senior manager for the IHS security research group in Austin, Texas, said in a phone interview.
Historically, terrorist attacks have spurred growth, and the Boston Marathon bombings also might have that effect, Everett said.
"As the event happened out in the open, the market may see more in the way of city surveillance projects," he said.
Although New York and some other cities have expanded the number of cameras using grants from the Department of Homeland Security, other cities such as Los Angeles have opted against widespread use of cameras, in part because of the cost, said Marc Rotenberg, executive director of the Washington-based Electronic Privacy Information Center.
"People have accepted some degree of public surveillance, but at the same time I just don't see a big push for a dramatic increase," Rotenberg said in an interview.
There are far fewer closed-circuit television cameras in U.S. cities than in London, where attacks by bombers seeking to drive the British out of Northern Ireland predate threats from Islamic extremists. London boroughs and the city's transit network together have deployed more than 91,000 CCTV cameras, according to 2011 data obtained by Big Brother Watch, a civil liberties group. More than 1,800 cameras monitored the athletic village during the 2012 Olympic Games hosted by the city.
There's no authoritative estimate of how many tens of thousands of other cameras are operated privately at stores, office buildings and other locations in the British capital, where closed-circuit cameras are easily visible on street poles, in shops, in the subway and on buses. In London, it's estimated that on average, an individual may be recorded by more than 300 cameras in a single day, according to a report last year by Big Brother Watch.
"London is probably the most surveilled city in the world," the group's director, Nick Pickles, said in a telephone interview.
In London, cameras helped track down criminals in riots across the city in August 2011, and the terrorists who attacked the bus and subway system in July 2005 were recorded on camera buying supplies for their bombs. In total, only one U.K. crime is solved for every 1,000 cameras because the majority of the 4.25 million CCTV cameras installed nationwide deliver low-quality images, according to a 2012 report by Bsria Ltd., a U.K. research and consulting company.
One law enforcement ambition is to couple real-time video with artificial intelligence software able to act before a terrorist bombing or other crime occurs.
Such systems would alert police to warning signs, such as an abandoned backpack or recognized face, in time to avert a potential terrorist attack.
New York, a target for terrorist plots more frequently than any other U.S. city, is advancing toward that capability with its so-called Domain Awareness System, an effort developed with Microsoft Corp. of Redmond, Wash., that's described as drawing real-time information from about 3,000 CCTV cameras and other sensors in lower and midtown Manhattan.
The camera network "now has the capacity to alert police to abnormalities it detects on the street, such as an abandoned package," said Bloomberg, who is founder and majority owner of Bloomberg LP, the parent of Bloomberg News.
In an interview on radio station WOR last month, before the Boston attack, Bloomberg remarked on the advance in surveillance technologies -- including police use of drones -- and its implications for individual privacy.
"You're going to have face recognition software; people are working on that," Bloomberg said March 22. "We're going into a different world, uncharted."
He added: "We're going to have more visibility and less privacy. It's not a question of whether it is good or bad. I just don't see how you can stop that."
Boston-area authorities had almost 150 cameras in the local network as of 2007, according to the most recent figures provided to the ACLU of Massachusetts. Carol Rose, the group's executive director, warns against what she considers the temptation to see increasing video surveillance as a solution to the terrorist threat.
"People have to understand there was a lot of surveillance at the finish line of the Boston Marathon," she said in a telephone interview. "Neither there nor in the many studies that have been done is there any evidence that surveillance is going to stop or deter someone from a violent act."
To the extent that surveillance can help to track a perpetrator, she said, "There is no objection to that so long as there are appropriate checks and balances.
"If we permit our fear to lead us down a path where everyone becomes a suspect, not only are we violating fundamental principles of democracy, but we also are undermining public safety because when everyone is a suspect, then no one is a suspect," she added.
Her concerns are shared by many Americans, according to a Washington Post poll after the Boston bombing. The poll found 48 percent of people worry that the government "will go too far" to investigate terrorism, versus 41 percent who said the government "will not go far enough."
The poll of 588 adults has an error margin of plus or minus five percentage points.
Government officials have considerable legal leeway to expand the use of video cameras in public places, said Christopher Swift, a lawyer and an adjunct professor of national security studies at Georgetown University's School of Foreign Service in Washington.
Many people misjudge the extent of their Fourth Amendment privacy rights outside the home, he said.
"Let's be honest here: The government can fly a helicopter over your house and hover there and take pictures of what's going on in your backyard, and the Supreme Court has said that is legal," he said in an interview. He was referring to a 1989 case, Florida vs. Riley, in which the high court held that police don't need a warrant to observe an individual's property from public airspace.
That ruling may have greater consequences than the justices realized at the time, as law enforcement agencies evaluate the possible use of small, unmanned aircraft in police work. Cities as small as Monroe, N.C., population 33,000, have been seeking to acquire drones.
Once prices drop, "People will think it's crazy not to use them for policing," said Stewart Baker, a lawyer at Steptoe & Johnson LLP and former assistant secretary for policy at the Homeland Security Department. "You can see how it would have been very useful to have drones fly over Boston during the manhunt."