US President Barack Obama directed his administration on Monday to aid Boston authorities in investigating the cause of what local reports said were twin explosions that rocked the finish line of the Boston Marathon.
A White House official said Obama had been notified of the "incident in Boston."
"His administration is in contact with state and local authorities. He directed his administration to provide whatever assistance is necessary in the investigation and response," the official said.
Separately, vice-president Joe Biden, in a telephone conference call about proposals to tighten gun laws, offered his prayers as he reacted to images he saw on television from Boston.
"As I'm speaking here, they just turned on the television here in my office. Apparently there has been a bombing. I don't know any of the details about what caused it, who did it. I don't think it exists yet," Biden said.
"But our prayers are with those people in Boston who have suffered injury. I don't know how many of them there are. I'm still looking at it on television now," he said.