WASHINGTON — President Barack Obama urged a nervous nation to avoid jumping to conclusions about Monday's explosions at the Boston Marathon, while acknowledging that officials "still do not know who did this or why."

Speaking from the White House just three hours after the explosions, Obama was cautious in his remarks, stopping short of calling the incident an act of terror. But he vowed to hold those responsible accountable.

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"We will find out who did this. We'll find out why they did this," Obama said in his brief, three-minute statement. "Any responsible individuals, any responsible groups, will feel the full weight of justice."

The president said the government would increase security around the United States "as necessary" but did not say whether his administration thought the incident was part of a larger plot.

Obama did not offer specific details on deaths or injuries, saying only that multiple people had been wounded, some of them gravely.

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