But the Boston Marathon is an unusually public event, giving hundreds of thousands of people easy access to the 26-mile route, and the bombs could have been set off by radical Islamists from the United States influenced by events or clerics in the Middle East, local extremists on either the left or right, or deranged killers with no ideological agenda.

Early reports suggest that the devices were crudely made — otherwise, they probably would have killed many more people — making it unlikely that they were the work of a foreign government or global terrorist group, such as Al ­Qaeda, the experts said.

The deadly twin explosions that shattered the Boston ­Marathon initially look more like the work of domestic terrorists than an international terrorist network, researchers and analysts who study terrorism said on Monday.


Radical Muslims “will be every­one’s favorite suspect, but there are many other possibilities,” said Brian Michael ­Jenkins, a terrorism specialist at the Rand Corp.

Emergency medical personnel transported the injured for treatment. David L. Ryan/Globe Staff

A State Police report in 2003 warned that the Marathon could be a “possible prime terrorist target” because it ­involves such a large number of runners and spectators, draws a live worldwide television audience, coincides with the Patriots Day holiday in ­Boston, and is shortly before the anniversaries of two other terrorist attacks, the Oklahoma City bombing on April 19 and the Columbine High School shooting on April 20.

“Terrorists around the world have traditionally sought large, symbolic, high visibility targets to attack,” the report said.

Violent Muslim radicals have been blamed for dozens of attempted strikes on the ­United States since the Sept. 11 attacks on New York and Washington in 2001, including the attempted car bombing in Times Square three years ago and the Fort Hood shooting in 2009 that killed 13 people.

Some media outlets reported a Middle Eastern connection in the Marathon bombing, too, saying that authorities were questioning a Saudi ­national being treated for burns at an area hospital.


But Boston police said they have made no arrests, and the FBI issued a statement ­Monday saying it “remains too early to establish the cause and motivation” of the attack, though the FBI agent overseeing the Boston office, Richard DesLauriers, called it a “potential terrorist investigation.”

Terrorism experts point out that domestic terrorists have been behind other attacks on US soil, including the Oklahoma City bombing that killed 168 people in 1995, the deadly Olympic Park bombing in ­Atlanta in 1996, and the ­Unabomber, who killed three people in more than a dozen ­attacks from 1978 to 1995.

Like most large public events, the Boston Marathon has long included a significant law enforcement presence with hundreds of officers. The ­efforts were strengthened after the Sept. 11 attacks to include additional air patrols, officers, and agencies. Even non­uniformed officers running the race were recruited to provide an extra set of eyes and ears.

State Police spokesman ­David Procopio said Monday that the event includes extensive security, including sweeps for explosives by bomb-sniffing dogs before and during the race.

But the Marathon may be impossible to completely ­secure, because it is held on public streets open to tens of thousands of pedestrians and spectators, in addition to the roughly 25,000 people who run the race every year. Over the weekend, the finish line was open to anyone strolling down Boylston Street in front of the Boston Public Library, including many who posed for pictures on the course in front of the spectator grandstands.


“There are an infinite number of targets in a developed society like the United States,” said Joseph Wippl, a professor of international relations at Boston University and a former CIA operations officer for 30 years. “No matter how much the government expends in the way of security, there will ­always be a chance that something like this happens.”

Juliette N. Kayyem, a Globe columnist and lecturer at ­Harvard’s Kennedy School of Government who has spent years working on counter­terrorism and homeland security, said she thought the bombing was probably the work of a terrorist in the ­Boston area who would appreciate the race’s importance to the local culture.

“It is probably home-grown, someone with a political cause from the right or the left,” said Kayyem, “someone who knows how appealing the Boston ­Marathon is to citizens here.”

“Even though it’s an international event, it is still a local event,” she said.

But Daniel Drezner, professor of international politics at Tuft University’s Fletcher School, cautioned that there is too little information to know who might be to blame. Some reports linked the Oklahoma City bombing to foreign terrorists, but it turned out to be the work of Timothy James McVeigh, an American seeking revenge against the government for its siege in Waco.

“Trying to speculate would be foolhardy,” Drezner said. “If anyone should learn anything from the past, it is that you shouldn’t speculate without more information.”


Todd Wallack can be reached at twallack@globe.com; Andrea Estes at estes@globe.com.