The two bombs at the Boston Marathon exploded with hundreds of people watching. Many of them were filming the moment on their mobile phones, which helped investigators identify, track and find the suspects.
Shortly after two bombs exploded in Boston, the police made their first call for assistance in their investigation by asking the public for photos or videos of the area around the finish line of the city's annual marathon. The images provided to the authorities helped determine the alleged bombers' identities within a matter of hours.
"It's an intensification of the classic call for witnesses," Cologne-based media expert Dietrich Leder told DW.
Unlike other witness statements, photos have the advantage that they remain reliable and don't forget - or incorrectly remember - what happened a few hours ago. Descriptions of suspects are often imprecise and witnesses often confuse the order of events or how long it took something to happen. A smartphone's camera, on the other hand, isn't susceptible to confusion and can be very important to investigations.
When cell phones crack the case
This was also the case in Boston. A mobile phone photo showing the marathon's finish area moments after the explosion captures one of the suspects among the people fleeing the scene. The person who took the picture provided it to the FBI, helping speed up the investigation.
There were also several surveillance cameras installed in the area of the explosion, many of which were installed to protect homes and businesses. Such cameras, however, deliver poor quality pictures.
"It's not a question of optics but of storage space," terrorism expert Paul Elmar Jöris said, adding that it takes a lot of space to keep a day or two's worth of video recordings. "Pictures from mobile phones or cameras, on the other hand, are of much better quality."
Other technical developments also helped investigators sift through the countless pictures they received.
"These days it's possible to use facial recognition software - a few years ago that would have been incredibly difficult," Leder said, adding that such programs are a major help to investigators who know what they are looking for. "Otherwise it would be crazy - you would have an amount of data you could never sort through."
Too many cameras?
While smartphones, surveillance cameras and TV video can help investigators, they can also pose problems as one of terrorists' aims is often to make their acts as public and visible as possible.
"Events with a major media presence do have a certain draw for terrorists," Jöris told DW.
Terrorists want to get as much attention as possible, said Rolf Tophoven, head of the Institute for Crisis Prevention (IFTUS) in Essen. But the experts also said they did not think terrorists choose a location because it has security cameras or because they think people will be making personal videos.
"Terrorists do not look for surveillance cameras but want a direct line to the media so they can be on people's televisions right away," Jöris said.
This, however, was not the case in the Boston Marathon bombs, Leder said.
"The explosions came several hours after the top runners were done," he said. "Most of the public had already turned their attention elsewhere. It was only private people who were still paying attention."
While experts can only speculate on what motivated the bomber or bombers, Leder said he was sure that the "public helped in the suspects' arrests." He said Boston police also used social networks, such as Twitter and Facebook, to inform the public, most famously a tweet saying "Captured!" when the second suspect was taken into custody.
Another side of digital investigations took place online, including on the Reddit discussion forum, where individuals looking for clues about possible bombers accused - and in some cases even threatened - innocent people who resembled the suspects.
"That's when things get very dangerous," Leder said. "Then we're looking at a form of digital mob justice that is very difficult to stop."
While authorities use online services to investigate crimes, terrorists also use them to plan crimes, Tophoven said. The bombers in Boston were reported to have followed bomb-making instructions from a website with ties to the al-Qaeda terrorist group.
"The Internet is like a university for jihad and terrorism," he said. "They can find everything they need to build bombs."