In the aftermath of deadly explosions at the Boston Marathon, two main questions --- who and why?
At the White House, President Barack Obama addressed the blast.
SOUNDBITE: U.S. President Barack Obama saying: "We don't yet have all the answers but we do know that multiple people have been wounded, some gravely, in explosions at the Boston Marathon."
In fact two people have been reportedly killed in the blasts... with dozens injured.
Police were put on alert in major cities across the U.S, including this beefed up security in New York's Times Square.
Obama vowed those responsible we be held accountable.
SOUNDBITE: U.S. President Barack Obama saying: "But, make no mistake, we will get to the bottom of this, and we will find out who did this, we will find out why they did this."
"Any responsible individuals, any responsible groups, will feel the full weight of justice."
After Obama's statement, the White House called the explosions an act of
