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, Virginia, 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.

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 6.6-meter 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 just kilometers 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.

By Wednesday afternoon, Patrick said in an interview Saturday, investigators had narrowed in on images of Dzhokhar Tsarnaev as the most likely suspect. “It was a remarkable moment when they narrowed in on Suspect Number 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. Officials said they went ahead with the public appeal for three reasons:

• Investigators didn’t want to risk having news outlets put out the Tsarnaevs’ images first, which might have made them the object of a wave of popular sympathy for wrongly suspected people, as had happened with two high-school runners from the Boston area whose photos were published on the front page of the New York Post under the headline “Bag Men.” At the news conference, FBI Special Agent in Charge Richard DesLauriers sternly asked the public to view only its pictures or risk creating “undue work for vital law enforcement resources.”

• During a briefing Thursday afternoon, President Barack Obama was shown the photos of the suspects by senior members of his national security team. Senior administration officials said that although Obama was not asked to approve release of the images by the FBI, the president offered a word of caution after viewing them. Be certain that these are the right suspects before you put the pictures out there, he advised his national security team, according to the administration officials.

• Investigators were concerned that if they didn’t assert control over the release of the Tsarnaevs’ photos, their manhunt would become a chaotic free-for-all, with news media cars and helicopters, as well as online vigilante detectives, competing with police in the chase to find the suspects. By stressing that all information had to flow to 911 and official investigators, the FBI hoped to cut off that freelance sleuthing and attend to public safety even as they searched for the brothers.

Facial-recognition software did not identify the men in the baseball caps, according to Davis, who said that technology came up empty even though both Tsarnaevs’ images exist in official databases: Dzhokhar had a Massachusetts driver’s license, the brothers had legally immigrated, and Tamerlan had already been the subject of some FBI investigation.

The FBI had had contact with Tamerlan at least as far back as 2011. Tamerlan, whose ethnic Chechen family immigrated to the United States in 2002, had indicated his interest in radical Muslim ideology both on Internet ramblings and with family and friends. On YouTube, Tamerlan created a playlist of videos titled “Terrorists.”

The brothers’ mother, Zubeidat Tsarnaeva, told Russia Today television on Friday that Tamerlan “got involved in religious politics five years ago.” She said FBI agents had been watching her son for “three to five years. They knew what my son was doing. . . . They used to tell me that he was really a serious leader and they are afraid of him.”

The FBI said Friday that at the request of a foreign government that was concerned that Tamerlan “was a follower of radical Islam and a strong believer,” the agency had interviewed him and his relatives but “did not find any terrorism activity, domestic or foreign.” Officials have acknowledged that the request came from Russia.

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 also spurred the brothers into action.

On Thursday evening, police responding to a robbery at a 7-Eleven in Cambridge, Massachusetts, examined surveillance video and noticed that in addition to the robber, the convenience store had been visited that night by two men who looked like the bombing suspects.

Then, shortly after a Massachusetts Institute of Technology campus officer was shot and killed, police got reports of an armed carjacking of a black Mercedes SUV nearby. The brothers had forced the vehicle’s driver to get them money from ATMs in the area. At a Shell 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.

The police commissioner said releasing images of the brothers may have spurred their violent spree. “We may have forced their hand by releasing the video,” he said. But he said that was nonetheless the right move: “I truly believe they were planning more attacks based in the arsenal I saw in Watertown. By forcing their hand we saved a much larger loss of life. . . . These individuals were bent on murder and mayhem.”

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 authorities lifted the stay-inside order Friday 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 to see that a human form was under the boat’s white plastic cover. They pounded the boat with flashbang grenades, a powerful concussive force, to see if the suspect would react; he barely did. Finally, an FBI negotiator on a bullhorn roused Dzhokhar and spent 25 minutes persuading him to come out. Local police cuffed the suspect, who was then taken by ambulance for medical attention to two gunshot wounds, likely suffered the previous night in the shootout with police.

After the arrest, Davis, exhausted but relieved, stood in the rain and looked back on four frenzied days of rugged, hurried and dangerous police work.

“Four days ago, my city was ruthlessly attacked,” he said. “There’s no explaining the savagery involved here. . . . I’ve spent the last several days looking at hundreds of hours of video tape. I got to see how brutal the attack was, over and over and over again.”

More important, he said, he watched how first responders and ordinary citizens put people back together. “Tourniquets,” Davis said. “Stemming the bleeding with their hands. Putting a man who was on fire out with their hands. These are the kind of things that came out of this savagery. It makes me proud.”