At the Cathedral of the Holy Cross earlier in the day, Obama declared to the people of Boston: "Your resolve is the greatest rebuke to whoever committed this heinous act." He spoke in almost mocking terms of those who commit such violence.
"We finish the race, and we do that because of who we are," the president said to applause. "And that's what the perpetrators of such senseless violence — these small, stunted individuals who would destroy instead of build and think somehow that makes them important — that's what they don't understand."
"We will find you," he warned.
Seven victims remained in critical condition. Killed were 8-year-old Martin Richard of Boston, 29-year-old restaurant manager Krystle Campbell of Medford, Mass., and Lu Lingzi, a 23-year-old Boston University graduate student from China.
Video and photos recovered in the investigation are being examined and enhanced by an FBI unit called the Operational Technologies Division, said Joe DiZinno, former director of the FBI lab in Quantico, Va.
Investigators are looking at video frame by frame — a laborious process, though one aided by far more sophisticated facial recognition technology than is commercially available, forensic specialists said.
The investigation will probably collect about a million hours of videotape from fixed security cameras and cellphones and cameras used by spectators, said Gene Grindstaff, a scientist at Intergraph Corp., a Huntsville, Ala., company that makes video analysis software used by the FBI and other law enforcement agencies.
But after years of investigating terrorist incidents and other crimes, the FBI is practiced at cataloging, categorizing and analyzing such evidence and will winnow it down dramatically, he said.
"Back in the days of 20 years ago, you were lucky if you had video and it was probably of poor quality and it took a tremendous amount of enhancement. Today you have a completely different issue," Grindstaff said.
Investigators can set the video analysis software so that it automatically searches for certain types of objects or people matching a height and weight description. The software can also spot patterns that analysts might not notice, such as a certain car that turns up in different places, Grindstaff said.
DiZinno, who ran the FBI lab from 2007 to 2010, said any retrieved bomb components such as the pressure cookers, shrapnel and pieces of timers or wire will be closely examined for fingerprints, DNA, hairs and fibers.
The bomb components would be traced by figuring out the item's maker, where each piece is typically purchased and whether the device resembles any bombs the FBI has seen in past attacks. The FBI lab keeps a detailed file on past bombings, including many overseas attacks.
"Let's say there was a timer," DiZinno said. "Was there a serial number? Who was the manufacturer? That can provide leads for investigators."
One pressure cooker maker, the Fagor Group in Spain, said that it has been contacted by U.S. investigators and that company officials are extending full cooperation. The company sells 250,000 pressure cookers a year in the U.S. and 1 million worldwide.
Link to FBI video: http://bit.ly/115ZcIq
Associated Press writers Jay Lindsay, Pat Eaton-Robb, Steve LeBlanc, Bridget Murphy, Meghan Barr, Jeff Donn and Julie Pace in Boston; Eileen Sullivan and Lara Jakes in Washington; Curt Anderson in Miami; and Marilynn Marchione in Milwaukee contributed to this report.