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BOSTON – 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 email 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,” said Boston Police Commissioner Edward F. Davis.

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 got a call from the 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 surgically removed at the knee, managed to eke out a request for pen and paper. In the intensive-care unit, 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.

Meanwhile, theories that developed via social media complicated the official investigation, according to law enforcement officials. Those officials said Saturday that the decision 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.

By Wednesday afternoon, said Patrick in an interview Saturday, investigators had narrowed in on images of Dzhokhar A. Tsarnaev as the most likely suspect. “It was a remarkable moment when they narrowed in on Suspect No. 2,” he said.

Law enforcement officials debated whether to release the photos, weighing the risk of the suspects fleeing or staging another attack against the prospect of quicker identification.

Once the photos of the men in caps were made public Thursday, the FBI tip line filled with calls, including one from the brothers’ aunt, who provided her nephews’ identity, according to federal law enforcement officials.

As investigators expected, making the photos public not only brought in new information, but spurred the brothers into action. That night, police responding to a robbery at a 7-Eleven in Cambridge, Mass., examined surveillance video and noticed that in addition to the robber, the convenience store had been visited by two men who looked like the two bombing suspects.

Then, shortly after a Massachusetts Institute of Technology campus police officer was shot and killed, authorities got reports of an armed carjacking of a black Mercedes-Benz sport utility vehicle nearby. The brothers had forced the vehicle’s driver to get them money from ATMs in the area. At a gas station in Cambridge, the security camera provided “extremely good video of two suspects,” a clear match with the photos from Boylston Street, Davis said.

In a violent confrontation with police in Watertown shortly after the carjacking episode, Tamerlan left the SUV and Dzhokhar, behind the wheel, tried to mow down police officers. In the process, he hit his brother, who was dragged under the car. Tamerlan died later that night. Police positively identified him by comparing his fingerprints against government records, Davis said.

After a tense day of searches on the silent streets of a locked-down city, David Henneberry was eager to get some air. As soon as the stay-inside order was lifted just before dusk, Henneberry stepped out of his Watertown house.

Something about his boat seemed off. The plastic cover was flapping in the wind, which made no sense, especially given that Henneberry had tied it down so well that it hadn’t moved even through this winter’s blizzards.

On inspection, the cover appeared to have been sliced open. Then Henneberry saw the blood. He came closer, pulled himself up a ladder to peer inside and saw more blood – and a curled-up form. He called 911.

Within minutes, he was hurried out of his house, and men in uniforms were firing at the boat and someone was shooting back.

Police had used thermal imaging technology from a helicopter to see that a human form was under the boat’s white plastic cover. They pounded the boat with flash-bang grenades, a powerful concussive force, to see if the suspect would react; he barely did. Finally, an FBI negotiator on a bullhorn roused him and spent 25 minutes persuading him to come out. Police handcuffed the suspect, who was taken by ambulance for treatment of two gunshot wounds, likely suffered the previous night in the shootout with police.