BOSTON — Within hours of the Boston Marathon bombing, investigators were already overwhelmed. Bloody clothing, bags, shoes and other evidence from victims and witnesses were piling up. Videos and still images, thousands of them, were beginning to accumulate.
Quickly, the authorities secured a warehouse in Boston's Seaport district and filled the sprawling space: On half of the vast floor, pieces of bloody clothes were laid out to dry so they could be examined for forensic clues or flown to FBI labs at Quantico in Prince William County for testing. In the other half of the room, more than a dozen investigators sifted through hundreds of hours of video, looking for people "doing things that are different from what everybody else is doing," Boston Police Commissioner Edward Davis said in an interview Saturday.
The work was painstaking and mind-numbing: One agent watched the same segment of video 400 times. The goal was to construct a timeline of images, following possible suspects as they moved along the sidewalks, building a narrative out of a random jumble of pictures from thousands of different phones and cameras.
It took a couple of days, but analysts began to focus on two men in baseball caps who had brought heavy black bags into the crowd near the marathon's finish line but left without those bags. The decisive moment came on Wednesday afternoon, when Massachusetts Gov. Deval L. Patrick (D) got a call from state police: The investigation had narrowed in on the man who would soon be known as Suspect No. 2, the man whom police captured Friday night bleeding and disoriented on a 22-foot boat in a Watertown driveway.
Patrick said the images of Suspect No. 2 reacting to the first explosion provided "highly incriminating" evidence, "a lot more than the public knows."
How federal and local investigators sifted through that ocean of evidence and focused their search on two immigrant brothers is a story of advanced technology and old-fashioned citizen cooperation. It is an object lesson in how hard it is to separate the meaningful from the noise in a world awash with information.
The killing of Tamerlan Tsarnaev and the capture of his younger brother, Dzhokhar, may seem like an inevitable ending given that their images were repeatedly recorded by store security cameras and bystanders' smartphones. But for 102 hours last week, nothing seemed certain in the manhunt that paralyzed a major metropolis, captivated the nation and confronted counterterrorism operatives with the troubling and unforgiving world of social media and vigilante detective work.
'Saw the guy'
While the analysts combed through videos frame by frame, a more traditional tip was developing two miles away at Boston Medical Center. Jeffrey Bauman, groggy from anesthesia, his legs just removed at the knee, managed to eke out a request for pen and paper.
In the intensive-care ward, Bauman, who had been near the finish line to see his girlfriend complete Monday's race, wrote words that would help lead to quick resolution of the bombings that killed three and injured 176 others: "Bag. Saw the guy, looked right at me."
