A day after two explosions at the Boston Marathon killed three people and injured more than 175, experts say it's not unusual that no one has taken responsibility for the attack.
Although President Barack Obama on Tuesday said that the attacks were being investigated as an "act of terrorism," authorities still don't know what motivated the attack, or whether foreign or domestic groups or individuals were behind it.
While some terrorist groups around the world have become famous by taking credit for bombings, terrorism experts said it is hardly a given that a group or individual will come forward.
"Historically, more than half of all bombings worldwide in fact go unclaimed," wrote professor Bruce Hoffman, the director of the Center for Security Studies at Georgetown who previously served as a counterterrorism adviser on the Office of National Security Affairs, in an email to The Huffington Post.
For example, the man behind the 1996 Atlanta Olympics bombing -- which resulted in 1 casualty and more than 100 injuries -- never claimed responsibility for the attack, and investigators identified him only after he was linked to a string of other bombings, Hoffman said.
In his research on why groups take credit for acts of terror, Aaron Hoffman, a professor of political science at Purdue University, found that organizations are more likely to take credit for an attack than "lone wolf" bombers, and when a group does claim responsibility, it is often motivated by competition from other terrorist groups. "Claim-taking is a form of advertising," he said in an interview on Tuesday with the Huffington Post.
"As the number of terrorist organizations increases, the more difficult it is for actual perpetrators to distinguish themselves from groups that had no hand in the violence," Aaron Hoffman wrote in a 2010 paper on the subject.
Groups are much more likely to claim responsibility than lone wolves, he said.
"Because groups have a larger purpose in mind which has to do with the health of the group itself, making sure that the group thrives, that the group improves its political standing, and is able to the be the focal point of organizing around issues the group cares about."
Still, Hoffman said, it is far too early to draw conclusions about the lack of a claim in the Boston attack. Sometimes claims come immediately or even beforehand -- Irish Republican Army members used to call police or journalists, and tell them when a bomb would go off -- but often claims come up to a week after the attack happens, he said.
Sometimes claims come years after the fact. Osama bin Laden didn't claim responsibility for the Sept. 11 until October 2004.
According to some research, claimed attacks are actually rarer than attacks where no one steps forward to take responsibility.
William Braniff, director of the National Consortium for the study of Terrorism and Responses to Terrorism, said that of 1,540 attacks between 1970 and 2011 in which data about claims of responsibility were available, just over one-fourth were claimed.
But he cautioned that this percentage might underreport the claimed acts of terrorism, if the claim never made the mainstream media. "I think the only conclusions one can draw from that data is that claiming of responsibility is not a given in terrorism in the United States," Braniff wrote in an email to the Huffington Post.
When asked for an average length of time between when an attack occurs and when a terrorist may claim the credit, Bruce Hoffman grew philosophical. "How long is a ball of string?" he wrote.