"Unless they kill themselves, they're going to get found," Marc Ehrlich said. He added: "There's nowhere in the world to hide."
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.