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.