Fueled by Twitter, online forums like Reddit and 4Chan, smartphones and relays of police scanners, thousands of people played armchair detective as police searched for men who turned out to be suspects Dzhokhar and Tamerlan Tsarnaev, ethnic Chechen brothers who had immigrated from southern Russia years ago.
But as amateur online sleuths began identifying possible culprits, caught in the virtual manhunt were people who were wrongly accused or placed under suspicion by crowdsourcing. It showed the damage that digital investigators can cause and raised a relevant question: In the social-media generation, what does law enforcement unleash when, by implication, it deputizes the public for help?
"The FBI kind of opened the door," said Hanson R. Hosein, director of the University of Washington Master of Communication in Digital Media program. "It was almost like it was put up as challenge to them, and they rose to it. ... They can be either really helpful or mob rule."
The bombings have been the highest-profile case in which the public has joined an active investigation, using ever-evolving crowd-sourcing tools, showing the pitfalls and benefits of new technology. It's certainly not vigilantism, but it's not standard policing, either. It's something in the middle, perhaps something new - the law-enforcement equivalent of citizen journalism.
As authorities asked the public for help, Reddit users began piecing together clues in the pictures and videos. They pointed out men who were wearing backpacks standing in the crowd. They looked at the straps of backpacks to compare with the one thought to have carried the bomb. They analyzed the bombs' blasts and people's gazes. In one particular photo thread, posted on imgur.com shortly after the bombing, a user attempted to pinpoint exactly where the bomb was placed.
"I'm seeing a lot of confusion and misinformation from news sites about where each bomb was detonated. After combing through the photos I've seen, I believe I've been able to make a solid case as to their exact location, where 'exact' in this case has an error margin of about 2 meters," user "gdhdshdhd" said.
Eventually, efforts and pictures posted in the sub-section dubbed "r/findbostonbombers" and elsewhere on Reddit were picked up by news organizations with more readers, giving it greater exposure.
Another wave of detective work happened after the FBI released pictures and videos of the Tsarnaev brothers and as initial reports of a shooting at Massachusetts Institute of Technology on Thursday night. While listening to the police scanner, Reddit and Twitter users thought they had heard the name of a Brown University student missing since March, and one user posted a news story about his disappearance.
That assumption proved wrong - and there was a cost.
The missing student's family, besieged with ugly comments, temporarily took down a Facebook page asking for help finding him. A few hours later, the online detectives said sorry - in the words of one moderator, "Rather Confused," for "any part we may have had in relaying what has turned out to be faulty information." Several Reddit users who posted on r/findbostonbombers did not immediately return messages seeking comment.
The president seemed to acknowledge this Friday night, speaking after the second suspect was captured. "In this age of instant reporting, tweets and blogs, there's a temptation to latch on to any bit of information, sometimes to jump to conclusions," Barack Obama said. "But when a tragedy like this happens, with public safety at risk and the stakes so high, it's important we do this right."
The rush to informal sleuthing began Monday soon after the smoke clear as pictures and videos from the marathon began to circulate on the Internet. Salah Eddin Barhoum, a 17-year-old track star who was a spectator at Monday's race, had his picture posted all over the Internet and ended up on the cover of the New York Post. He told The Associated Press on Thursday that he is now afraid to leave his house.
Some of the amateur police work didn't sit well with the professionals. Boston's police department, for example, has a very active Twitter account with more than 220,000 followers, but the onslaught of misinformation proved to be too much. At one point, Boston police asked people to stop tweeting information from their scanner traffic.
Other police departments have faced similar situations. In the Seattle area, journalists and members of the public were asked to stop tweeting as authorities looked for a man who killed four police officers in 2009. In Los Angeles, authorities had the same request as they looked for rogue ex-police officer Christopher Dorner earlier this year.
"It's completely human nature, it's to be expected that people are going to take events and try to apply meaning," said Sgt. Sean Whitcomb, spokesman for Seattle's police.
The Seattle Police Department has fully embraced Twitter and blogs, using both frequently. Whitcomb said the department disseminated as much information as it could, such as pictures, on two recent cases in which there were armed suspects on the loose that sparked brief citywide manhunts.
"We also want to make sure we are having a voice in the conversation," Whitcomb said. "We want people to go to us first and cover the information we're putting out. If we can get that done, it's a win."
While Reddit and 4Chan have been around for several years, their prominence has grown of late. More and more news organizations have learned to use them to mine information. For Hosein, sub-sections on Reddit have become something like local newspapers, except it's the users providing the content.
"Citizens think they almost have an obligation to rise up to do the work," he said.
Hosein says that the FBI's call for help was no different than a "Wanted: Dead or Alive" poster from the 1800's - albeit with much more amplification and distribution. But he feels that after this week's saga, people will eventually learn to exercise caution.
"There's a sense that we're learning collectively quickly, that we actually have to take on some of the sourcing rules that journalists have had in the past," Hosein said. "I've seen more restraint like, 'Wait guys, hold on, there's gotta be more confirmation.' I know we're learning. I don't think it's going to be repeated."