Investigators swiftly obtained a vast quantity of amateur photos and videos taken by onlookers, often with their cell phones, as well as extensive footage from surveillance cameras in the area of the blasts. The FBI released images Thursday from one of those cameras, zeroing in on two men in caps who have become the suspects in the case. They're seen walking together; the FBI said one of them later set down a backpack where the second explosion occurred.
If indeed the video provides the crucial break in the Boston case, surveillance cameras -- which have proliferated in London, Chicago and elsewhere -- may take on new allure. Informal surveillance by private citizens may proliferate as well; the FBI says it expects the public to be its "eyes and ears" as the investigation continues.
The upside of this expanding surveillance network is clear -- a greater potential for law enforcement to solve crimes and, in some instances, to prevent them. David Antar of New York-based IPVideo Corporation says video surveillance can be set up to trigger warnings if bags are left unattended or suspicious activity takes place before or during a large-scale event.
Is there a downside?
Some civil libertarians say yes. While they welcome any tools that can help solve a crime as brutal as the bombings, they worry about an irrevocable loss of privacy for anyone venturing into public places.
"It's now harder and harder to go about our lives without being tracked everywhere," said Ben Wizner, a lawyer with the American Civil Liberties Union who specializes in privacy and technology issues.
"The ACLU doesn't object to cameras at high-profile public places that are potential terrorist targets," he said. "What we do object to is a society in which cameras are so pervasive that we can't go about our lives anywhere without them being recorded and stored in data bases forever."
Within the past decade, the scope of surveillance -- both private and government -- has increased incalculably. Just this week, security video helped lead to the arrests of a Texas couple in the slayings of two prosecutors and the wife of one of them. Authorities say the accused man, a former justice of the peace, was embittered because he had been convicted of stealing government computer monitors -- an offense which was itself captured on security video.
And then there is self-surveillance. Millions of people check in regularly with Foursquare to communicate their whereabouts; many millions more passively enable themselves to be tracked simply by carrying their cell phones.
Photographs and videos can rocket through cyberspace, instantly viewable by strangers on the other side of the world or by law enforcement agencies, courtesy of Instagram, Facebook, Twitter and other social media.
Attitudes toward surveillance and privacy may be shifting. There's a generation of teens and young adults who have grown up with social media and may be more reconciled than older Americans to the prospects of being tracked.
"Americans still cite privacy as one of the core values they cherish, but what's happening is this slow, insidious erosion of it," said Jonathan Turley, a law professor at George Washington University.
"Humans need at times to feel they can exist freely and without constant observation -- it is essential to our right to association and expression," he said. "And yet we have a generation being raised in a fishbowl society. They're more tolerant of government surveillance, and that can be a danger to a free society."
Compared to the United States, surveillance cameras are far more pervasive in Britain, where they were first used decades ago to protect against attacks from Irish militants. Up to 4 million or so cameras are now in place, including some around the house of George Orwell, the author of "1984," which foretold of a "Big Brother" society.
Among the British public, the cameras seem to be widely accepted -- especially in the aftermath of the 2005 suicide bombings that killed 52 commuters during morning rush-hour traffic in London. Evidence from closed-circuit cameras helped crack that case.
"If you're not doing anything wrong, you have nothing to be worried about," said Joseph Clarke, 32, a London banker. "I'm out all of the time and I don't even notice them. We need them."
Nonetheless, a London-based organization called Big Brother Watch has been campaigning to cut back on the surveillance network.
"While it provides a sometimes useful tool after an event, it doesn't address the root causes of crime and doesn't protect the public," said the group's director, Nick Pickles. "The public has been desensitized, and so have the perpetrators of crime. The initial deterrent effect has largely disappeared because people just take it for granted."
Last year, Big Brother Watch issued a report revealing that more than 200 high schools had installed surveillance cameras in bathrooms and locker rooms. School officials defended the initiative, saying it was needed to combat bullying and did not reveal images of students using the toilets.
In the United States, Chicago has the most comprehensive network of surveillance cameras, estimated at more than 10,000. They are mounted on street poles and skyscrapers, aboard buses and in train tunnels; the rail system alone has more than 3,600 cameras.
Police credit the network for thousands of arrests in recent years. Cameras have recorded drug deals, bike thefts and other crimes; footage from a camera on a city bus helped convince a suspected gang member to plead guilty to shooting a high school student in 2007.
After the Boston Marathon bombings, Mayor Rahm Emanuel was quick to tout Chicago's surveillance cameras.
"They serve an important function for the city in providing the type of safety on a day-to-day basis -- not just for big events like a marathon," he said.
Police say they get few complaints about the network. And even the local branch of the ACLU says Chicagoans generally seem at peace with the system -- except when they get a traffic ticket for a camera-recorded infraction.
Police have not always had their way in expanding surveillance networks. In Washington, D.C, the city council balked at appropriating money in 2008 for a network of more than 5,000 cameras after privacy and civil liberties groups campaigned against the plan.
Attorney Hanni Fakhoury of the Electronic Frontier Foundation, which advocates for online free speech and privacy rights, said the data amassed by police from surveillance cameras and personal devices has enormous crime-solving potential. But he said there were worrisome questions about how long such data would be stored, and who could access it.
"There seems to be a suggestion, that just by walking in a city square, you give up your rights to be anonymous," he said. "We could stop all sorts of crime ahead of time if we monitored everything everywhere. But do we want to live in that kind of society?"
Associated Press writer Paisley Dodds in London contributed to this report.
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