As investigators raced to figure out who was behind the bombings of the Boston marathon this week, one of the clues was the site itself.

By considering the location of an attack, also known as the "target," experts can gain insight into the motivations of the perpetrators and their level of sophistication.

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"It's part of the puzzle when we look at the type of devices used, the pattern of explosions, the materials associated with the explosions, the timing and sequencing of explosions," said Randall Rogan, an expert in crisis negotiation and terrorism at Wake Forest University.

"The biggest indication (of the target) is what the motive was," he added. "This was done to create as much fear and terror and insecurity as possible in the civilian population and to instill a sense of vulnerability."

Experts often refer to two types of targets -- "hard" and "soft." Hard targets are highly patrolled and often symbolic locations, such as military bases, embassies and government buildings.  Hard targets are difficult to attack successfully, Rogan said, and hits on them represent an expression of highly organized power and aggressiveness.

Soft targets, on the other hand, refer to unprotected or difficult-to-protect environments that are full of ordinary people -- places like busses, restaurants and sporting events, including marathons. Soft targets are easier to access than hard targets are, and attacks on them more likely to impact the civilian population, both physically and emotionally.

"When you attack a soft target, you're saying, 'You as civilians are not safe anywhere,'" Rogan said. "You are vulnerable at any given point in time."

Boston Police look at blown out windows at the scene of the first explosion on Boylston Street near the finish line of the Boston Marathon.Boston Globe via Getty Images

The scale of an attack can also reveal details about who was behind it, said Anthony C. Roman, CEO of Roman & Associates, a private investigation and security consulting firm in Lynbrook, N.Y.

The 9/11 attacks on the World Trade Center, for example, required significant amounts of planning, large numbers of people engaged in complicated logistics, and a sophisticated command-and-control structure that employed covert communication. The grand goal was to disrupt government, the economy and our daily lives, while also spreading fear. Such an attack had to be the work of a major terror group.

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In contrast, Roman said, smaller-scale attacks on soft targets like a marathon can be pulled off with no organizational structure, little money and information about bomb-making pulled off the internet. That leaves open the possibility that a single deranged person planted the bombs in Boston for reasons that are still unclear.

The marathon bomber, in other words, could be the next Timothy McVeigh or Anders Breivik. Equally possible, though, is that a terrorist group wanted to shake things up.

"When you look at the marathon as a soft target and the bombs as reasonably unsophisticated, it can be the work of an individual terrorist sympathizer with no known affiliations, it can be a homegrown domestic terrorist group or it can be a sophisticated international terrorist group that has changed its tactics," Roman said. "It's really difficult to tell at this point."

Though the site of the attack is not enough on its own to reveal who the attackers were, Roman said, the target has factored in to the investigation, along with the materials and methods used to construct the bombs and other evidence, including whether similar attacks follow.

"If we see a pattern," Roman said, "it would be a clue that we have a serious problem."