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BOSTON — Investigators are convinced the explosive devices that killed three people at the Boston Marathon were not in place before the race began because of sweeps by security teams. And since Monday they have focused on increasingly narrow slices of the five-hour window between when the race started and the blasts occurred, according to U.S. officials.

This methodical dissection of terabytes of video, still images and other evidence led the FBI to images of two potential suspects Wednesday, officials familiar with the investigation said.

The apparent breakthrough illustrates how private surveillance equipment, in combination with the cellphone cameras used by ordinary citizens, has become an extraordinary resource that allows investigators to re-create the visual narrative of the streetscape surrounding a location in order to scrutinize the hours, minutes and seconds ticking down to a crime.

“There is absolutely going to be video of almost every single inch, for every single second of that day; it’s just a matter of finding it,” said Andrew Obuchowski, a former Massachusetts police officer who analyzes video evidence for Navigant Consulting, a private firm.

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Authorities have obtained clear images of the faces of two men with backpacks who they believe were acting suspiciously around the time of the bombings, a potential breakthrough in the search to find who planted the deadly devices, sources familiar with the investigation said Wednesday.

A department store’s surveillance camera caught an image of at least one of the men leaving a backpack near the finish line, a federal law-enforcement official said.

Another official briefed on the investigation said the image that shows two men is the first indication that more than one bomber may have been responsible for the attacks that killed three people and injured more than 170 at Monday’s race.

The men were singled out because of their demeanor and the way in which they reacted to the bomb blasts, said these officials, who could not be identified because they were not authorized to discuss the investigation publicly.

Neither would say how close authorities were to identifying the two.

The photographic evidence adds to physical evidence already gathered, including parts of a pressure cooker probably used in the two bombs that went off as hundreds of runners were still streaming in five hours into the race.

Authorities are relying not only on extensive surveillance video but a flood of photos and videos sent in by spectators, office workers and others who were at the disaster scene near Copley Square.

“I think that this will go down in U.S. history as the most videotaped bombing in history,” said Tom Thurman, who formerly headed the FBI’s Bomb Data Center and helped investigate the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, and the 1993 bombing of the World Trade Center.

President Obama was to speak Thursday at an interfaith service here for the victims, 59 of whom are still hospitalized, with 10 in critical condition.

The disclosures about the photos emerged on a dizzying day of conflicting reports in which a number of news agencies initially reported a suspect had been identified and arrested, and was supposedly scheduled for a court appearance.

Hundreds of reporters and spectators gathered outside the Joseph Moakley federal courthouse near Boston’s Seaport district, which was briefly evacuated for a bomb threat, even as the FBI and the Boston Police Department denied anyone had been taken into custody.

CNN, Fox News Channel and The Boston Globe said a suspect in Monday’s bombing had been arrested. The Associated Press said a suspect had been taken into custody. Within an hour, the FBI denied a suspect had been captured, leading the three news organizations that had reported the arrest to back down from those claims.

The AP, while reporting the federal denial, said its original source was standing by its claim that a suspect had been taken into custody. The news cooperative said its source was a law-enforcement official speaking on condition of anonymity.

By nightfall, with no evidence that anyone was in custody, the original source was unable to further explain what was going on.

ABC, CBS and NBC all broke into their regular programming to report progress in the case, but did not say there was an arrest or someone brought into custody.

CNN’s John King had jumped out early around lunchtime, saying a department store’s surveillance camera had helped law enforcement spot a person dropping a container on the street that was believed to be the second of two bombs that detonated near the race’s finish line.

King reported at 1:45 p.m. that an arrest had been made. The Boston Globe tweeted the same thing at 1:53, attributing it to an unnamed official. Six minutes later, the Globe sent out a second tweet, saying CNN was the source of its arrest report. Fox News Channel’s Megyn Kelly said at 1:55 that the network had been told of an arrest.

The Associated Press sent out a NewsAlert at 1:53 saying that an arrest was imminent. At 2:14, The AP said a suspect had been taken into custody, but did not say there was an arrest.

The three biggest broadcast networks jumped into the story with cautious reports of progress within five minutes of each other shortly before 2 p.m.

NBC reporter Pete Williams was insistent that news organizations reporting an arrest had jumped the gun.

“From the beginning of this, this has been the hallmark of this story — information going in totally different directions coming from normally very reliable sources,” Williams said. “We can’t just flip a coin on this.”

CNN spokeswoman Barbara Levin noted that the network had three credible sources on the local and federal levels for King’s initial report.

“Based on this information, we reported our findings,” she said. “As soon as our sources came to us with new information, we adjusted our reporting.”

The FBI scolded news media outlets Wednesday for mistakenly reporting an arrest and warned that such unverified reporting could have “unintended consequences” for its investigation.

The FBI issued a statement later in the afternoon: “Over the past day and a half, there have been a number of press reports based on information from unofficial sources that has been inaccurate. Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting.”

Judy Muller, a former network news correspondent who teaches journalism at the University of Southern California, wrote in an email: “ I fear we have permanently entered the Age of the Retraction. All the lessons of the past — from Richard Jewell to NPR’s announcement of the death of Gabby Giffords to CNN’s erroneous report on the Supreme Court Ruling on ObamaCare — fail to inform the present. The rush to be first has so thoroughly swallowed up the principle of being right.”

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