Also, the fact that the bombs were close by, and went off within moments of each other, suggests that "somebody knows what they are doing," Ryan said. "This is not the work of an amateur."
Ryan likened the Boston bombings to the Madrid train bombings that were later linked to an Al-Qaeda group.
"Anti-tax protestor groups wouldn't likely go after a crowd of Americans enjoying themselves in a running race," Ryan said. "That would be a misdirected target."
However, another terrorism expert said figuring out the authors could be difficult. Al-Qaeda may have been the worst enemy a few years ago, but that list has gotten bigger.
"This is the kind of attack that is calculated or linked to an event," said Bruce Hoffman, director, Center for Peace and Security Studies at Georgetown University. "That means whoever did it wants attention. There will be some kind of claim of who is behind it."
Hoffman believes anti-government groups in the United States could be behind the bombings, especially given the divisive nature of the current national debate over gun control, the administration's drone program and even news about a hunger strike by inmates at the Guantanamo prison.