Advances in image analysis empower law enforcement but worry privacy advocates Advances in computing power and analytical software have allowed for individuals to be identified and tracked as never before - especially when that information is combined with the location data emitted by most cellphones. The role of these technologies in the Boston investigation is unclear, but law enforcement experts say that video surveillance and analysis is especially valuable in cases such as this, with an unexpected attack and no obvious suspects at first. The process of winnowing happened with remarkable speed, moving in just days from a mountain of unsorted video, to blurry images of potential suspects, to pictures crisp enough to drive a manhunt. For those seeking to protect privacy in the digital age, however, the news is not all good. Computer analysis of the faces of those who apply for driver's licenses, passports or entry visas already has created vast databases that some law enforcement officials are eager to use on a routine basis, for what amounts to digitized lineups of tens of millions of people. The debate over how to balance the needs of investigators with the rights of private citizens remains unsettled. Civil libertarians worry that data gathered for one investigation will inevitably be used for other purposes, allowing a gradual slide into perpetual surveillance of private citizens. There are so many video cameras operated by so many entities, public and private, that no one has a credible count of how many are in use, though estimates in lower Manhattan alone top 3,000, said Jennifer Lynch, a staff attorney with the Electronic Frontier Foundation. "Somebody who lives and works in that area could be surveilled in almost everything they do," she said. That is a society that most of us do not want to live in. Analyzing video has been key to bombing investigations worldwide and was especially key to unraveling the Islamist bombings in July 2005 in London, perhaps the city with the world's most extensive network of video surveillance. Cameras were key even as long ago as April 1999, when a bomb injured dozens of people at a market in Brixton, a district in south London, said Hugh Orde, a top police official during the investigation. "We had, from a detective point of view, a disaster," he recalled. But by reviewing surveillance videos, police identified a man wearing a cap low on his head and carrying a bag. In another clip, the bag was gone. The pictures broke open the case and led to an arrest. Facial-recognition technology used by the FBI was first developed during the Iraq War to search for suspects in the aftermath of bombings, which typically featured crude, handmade explosives resembling those used in Boston.