Who did it? That's the question on everyone's mind after two explosions at the finish line of the Boston Marathon on Monday killed three and injured 176 people. Boston police say they uncovered two additional homemade bombs that were also set to go off. There also was an explosion at the JFK Library just south of downtown Boston that may or may not have been related to the others.

Massachusetts Gov. Deval Patrick called the bombings "an attack" during an afternoon press conference. As details become more clear, some terrorism experts say the explosions carry the hallmark of a foreign terrorist group.

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"The list of suspects is long; any number of people have an axe to grind against the United States," said Ken James Ryan, a terrorism expert at Cal State University, Fresno. "It could be any number of people: Al-Qaeda and North Korea, or any other wannabee organizations. Who knows? At this point someone will take credit for it."

Even though Monday is tax day in the United States, an important anniversary for domestic anti-government or anti-tax groups, Ryan believes that the attack was committed by a foreign group. First, the planting and detonation of more than one bomb takes the kind of technical know-how and desire to hurt and kill a large number of people.

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."

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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."

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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.

"We have no shortage of Americans wanting to kill Americans for idiosyncratic reasons," Hoffman said. "In this case, one doesn't have to speculate too far to see a political motive."

Hoffman said Americans have been lucky in the post-9/11 years not to have had more such attacks, whether the culprits originated inside or outside of U.S. borders.

"We are less safe than we imagine we are, unfortunately," he said.