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Boston is one of the less-wired large cities when it comes to surveillance cameras, so authorities relied largely on footage from private parties, such as the Lord & Taylor department store near the scene. The most recent estimate, from 2010, is that Boston and surrounding towns have some 150 police surveillance cameras, plus 400 in the subway. This compares with more than 3,000 government and networked private cameras in New York City's financial district alone, and some 400,000 cameras in London.

The cameras are getting smarter. New software goes beyond passive recording to alerting law enforcement about suspicious activity in real time. Video analytics enable what is called "activity forecasting." By applying artificial intelligence to video, these services issue alerts of what researchers call "anomalous" behavior--such as when cameras detect people leaving bags behind in public places.

The technology, from companies with names like IPVideo Corp. and ObjectVideo, is still new. It might not have been good enough to have identified the bags left behind by the terrorists in time to disarm the Boston bombs. As these systems improve, however, there will be a growing gap between cities that make full use of surveillance technologies and those that don't.

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