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“Our prayers are with those people in Boston who have suffered injuries. I don’t know how many there are,” Vice President Joseph R. Biden said.

An analyst warned that the explosions were likely terrorist bombs and more could be coming.

“The apparent Boston Marathon bombings are likely a terrorist attack,” said Ben Venzke of private-sector intelligence outfit IntelCenter.

Law enforcement sources told NBC that the first of two explosions appeared to be caused by a small homemade bomb, perhaps hidden in a mailbox. The second explosion also makes it likelier that both were deliberate.

Mr. Venzke said he was closely monitoring extremist websites and chat rooms for any claim of responsibility or threat of further attacks.

“No credible claims have been identified so far,” he said, but if the perpetrators were a properly organized and experienced terrorist group, “there is the distinct possibility of additional coordinated attacks in Boston and/or other major cities.”

Sen. Dianne Feinstein, California Democrat and head of the Senate Select Committee on Intelligence, shot down news reports that said police were holding a Saudi national in connection with the explosions.

Reminiscent of 9/11

Across the country, viewers who tuned into daytime TV programming such as the “Ellen DeGeneres Show” and “The Dr. Oz Show” were greeted instead by urgent special reports on what had happened at the race — one of the city’s biggest events, held each year on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.

Competitors and race organizers were crying as they fled the chaos. Bloody spectators were carried to the medical tent that had been set up to care for fatigued runners.

Ryan Stevenson, 29, who lives outside the city, said he found out about the blast shortly before leaving work to walk to the North Station transportation hub.

“But even if I hadn’t heard, you could tell that something was wrong on people’s faces,” he said in an email to The Washington Times. “Runners wearing their medals looked distant to me. Parents holding their children had a sickened expression on their face.”

Police scanners lit up with crosstalk about suspicious packages and what to do with personal effects the runners had to leave behind, while pro hockey officials postponed a game in Boston between the Ottawa Senators and hometown Bruins.

Initial reports of a third explosion at the John F. Kennedy Presidential Library and Museum were investigated, though later reports suggested it was an unrelated fire at the building.

Massachusetts Gov. Deval Patrick said Mr. Obama assured him that the state would have the full cooperation of the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

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