Counterterrorism specialists said the authorities would aim to match the faces of any possible suspects, using facial recognition software, against an array of databases for visas, passports and drivers’ licenses. “It’s our intention to go through every frame of every video that we have to determine exactly who was in the area,” Edward Davis, the Boston police commissioner, said at the news briefing. “This was probably one of the most well-photographed areas in the country yesterday.”

Boston was deserted on Tuesday morning, not only because many of the runners and spectators were leaving town, but also because yellow police tape and metal barriers still marked off a nearly mile-long area encompassing the two explosion sites, one that the police described as the most complex crime scene they had ever encountered.

At the morning commuter rush, the city’s subway system was uncharacteristically quiet, watched over by the police and SWAT teams. Stores on Newbury Street, Boston’s busy retail thoroughfare, were closed, and tables on the patio at Stephanie’s, a restaurant there, were still covered in dishes left there on Monday.

Among the three dead was an 8-year-old boy, Martin Richard of Dorchester. The boy had been watching near the finish line and then moved back into the crowd; the blast killed him and severely injured his mother and his sister.

Another spectator, Krystle Campbell, 29, of Arlington, Mass., also died Monday from injuries she suffered while watching the marathon, her grandmother Lillian Campbell said Tuesday.

The third person who died was identified by Boston University officials as a graduate student there, and the Chinese Consulate in New York said that she was a Chinese national. The university is waiting for permission from the family before releasing her name. She was watching the race close to the finish line, saidRobert Brown, president of the university, in an e-mail to the university community.