Within hours of the Boston Marathon bombing, investigators were already overwhelmed. Bloody clothing, bags, shoes and other evidence from victims and witnesses was piling up. Videos and still images, thousands of them, were pouring in by e-mail and Twitter.
Quickly, the authorities secured a warehouse in Boston's Seaport district and immediately filled the sprawling space: On half of the vast floor, hundreds of pieces of bloody clothes were laid out to dry so they could be examined for forensic clues or flown to FBI labs at Quantico, Va. for testing. In the other half of the room, more than a dozen investigators pored 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.
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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 Patrick 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.
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."
FBI agents quickly came to Bauman's bedside. A man in sunglasses and black baseball cap had walked right up to him, placed a black backpack on the ground and stepped away, Bauman remembered.
His tip became a critical lead, according to law enforcement officials.
Of course, investigators had 2,000 other leads, too, in the form of photos and video that "almost became a management problem, there was so much of it," said Davis, who led the local piece of the probe from a ballroom at the Westin Hotel where 100 officers and commanders from local, state and federal law enforcement collaborated. The room was equipped with tables for laptops, power strips and, most important, land lines, since cellphones were unreliable in the chaos after the bombings and satellite phones worked only if you stood by a window.
Davis had learned of the central importance of video from a police commander in London after the public transit bombings there in 2005, when the city's extensive system of surveillance cameras led to identification of four suspects within five days of the attacks, after examination of hundreds of hours of video.
Eight years later, the social media revolution meant that the FBI and Boston authorities were under intense pressure to move even faster, because thousands of amateur sleuths were mimicking the official investigation, inspecting digital images of the crowd on Boylston Street and making their own often wildly irresponsible conclusions about who might be the bombers.
On an investigative forum of Reddit.com, since removed from the site, users compiled thousands of photos, studied them for suspicious backpacks and sent their favorite theories spinning out into the wider Internet.
"Find people carrying black bags," wrote the Reddit forum's unnamed moderator. "If they look suspicious, then post them. Then people will try and follow their movements using all the images."
The moderator defended this strategy by arguing that "it's been proven that a crowd of thousands can do things like this much quicker and better. ... I'd take thousands of people over a select few very smart investigators any day."
In addition to being almost universally wrong, the theories developed via social media complicated the official investigation, according to law enforcement officials. Those officials said Saturday that the decision on Thursday to release photos of the two men in baseball caps was meant in part to limit the damage being done to people who were wrongly being targeted as suspects in the news media and on the Internet.
That decision, which appeared to be a straightforward request for the public's help in identifying the two men, turns out to have been a tactic with several purposes.
As investigators reviewed images, the young men in the black and white baseball caps came to stand out from the rest, Davis said.