Investigators in Boston have started the task of poring over extensive images from the city's vast collection of security cameras for clues about who planted deadly bombs near the finish line of Monday's Boston Marathon.
Authorities also appealed to the public for amateur video or photos that might provide information about the two blasts that killed three people and wounded 150 others.
Numerous American cities, including Boston, have installed security cameras at key public vantage points in hopes of preventing terrorism and to provide evidence about other crimes.
Security cameras are installed in Boston's bustling shopping districts, such as the city streets where the bombs exploded, and along major thoroughfares.
Boston has nearly 500 cameras on its subway system, and more on city buses. Cameras on bridges and at toll booths could trace anyone driving through those vantage points.
The cameras are linked to the city's Real Time Crime Center, where police can monitor activity across a large swath of the city.
The system links Boston and eight nearby communities so authorities can share images and possibly track the movements of suspects.