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As Boston bombing photos and videos pour in, where do investigators begin?

People look on and take photos of the police, fire and SWAT crews following the Boston marathon bombing on April 15, 2013.
People look on and take photos of the police, fire and SWAT crews following the Boston marathon bombing on April 15, 2013.Charlie Mahoney/Prime / NBC News

When bombs went off at the Boston Marathon finish line Monday, there were nearly as many camera-equipped smartphones as people there in Copley Square. It could well have been the "most photographed spot on Earth at that moment," as Slate speculated. Now, the Boston Police Department and the FBI are hoping to collect all  that crime scene data.

"What we would like is forwarding any photos that you have around the time of the blast, particularly in that area," Boston police commissioner Ed Davis said at a press conference Tuesday, encouraging people to email photos to Boston@ic.fbi.gov.

"Tell us what time the photo was taken ... so we don't have to go through those electronic signatures, [so] we have some data as to when these photos were taken." Photos taken just before, and just after after the blasts would be most critical to the investigation, he said.

What, exactly, are the cops looking for?

"Whether there's going to be someone who is wearing clothing that's suspicious on a relatively warm day, or carrying a large backpack — those would be sort of smoking guns," Michael Leiter, former director of the United States National Counterterrorism Center, said in an on-air interview on MSNBC.

The trouble is, without more clues, the images may yield little. "What they're probably going to be looking for is anyone who looks a little bit suspicious, and frankly, they dont know what that is at this point. And that's why it's going to be such painstaking work," Leiter said.

Most crucially, "a lot of what shows up on these photos and videos won't seem relevant until they get additional information from someone else. This is going to take hundreds and hundreds of man hours."

If even a fraction of eyewitnesses sent in a few photos or their own Zapruder-style video, investigators stand to receive a deluge of digital files. So where do they start when dealing with a photo and video evidence bin this big? And what, if anything, will the Internet's "crowds" — like those now currently surfacing their own bombing images on Reddit — add to the investigation?

Once the police figure out what "suspicious behavior" looks like, they can enlist the help of computers to scan images and videos for certain descriptors, such as "brown hair" or "yellow jacket" or "caucasian."

"You can enter these attributes, and [the software] builds a visual model of the target and then scans for the pattern," Lorenzo Torresani, assistant professor at the Vision Learning Group at Dartmouth, told NBC News. 

In the approach Toressani created, images are broken down into units, sort of like tag words, that can be called up, anything from vague ("backpack") to specific ("Dalmatian dog"). The software can comb through millions of photo files in under a minute.

When we asked the FBI if they used this software, a spokesperson told us it doesn't discuss the brands or types of computer systems that it uses, but Torresani "suspects" that software like this is available to the criminal investigators.

When facing a crowdsourced data trove, there's also the possibility that some images are fake.

Image authentication software currently in use by law enforcement agencies, developed by software startup Fourandsix, can read the JPEG fingerprint of an image — unique to the device that made the image — to call out images that were altered by image editing software like Photoshop. Sometimes it's not always apparent what is altered, but it flags images that might be off. "You can think of it as a triage," Hany Farid, co-founder of Fourandsix (and Dartmouth computer imaging professor), told NBC News.

As the FBI and Boston police continue to collect eyewitness photos and video from tipsters, armchair investigators across the Web are starting their own searches. Redditors began posting even before the second explosion went off, Matt Lee, the main moderator of r/Boston, told NBC News. "We had a ton of posting ... people asking further questions," he said.

And as the images and information poured in, the crowds on Reddit began parsing the data.

"The community is generally good at sifting out [bad information]," Lee says. "People are critical of what other people post." In the past, keen-eyed Redditors have called out such hoaxes as the Puerto Rican street sharks and explained why such viral images were fake.

Meanwhile, Twitter was momentarily captivated by a person on a roof, a detail in one of the photos that circulated after the explosions. While there's a sliver of an outside chance that this person may have had something to do with the crime, those playing along are better off following the leads of the authorities. 

Sure, amateur sleuths in other fields like astronomy have lucked out with great finds in the past, but in those situations, the folks at home had all the data in the universe in laid out in front of them.

Nidhi Subbaraman writes about technology and science. Follow her on Twitter and Google+.