President Obama: 'All Americans Stand With The People Of Boston'

President Barack Obama, responding to the explosions at the Boston Marathon, says the United States does not know "who did this or why" but vowed that whoever is responsible "will feel the full weight of justice."

He said: "We will find out who did this and we will hold them accountable."

Obama made his remarks Monday evening from the White House about three hours after two explosions detonated near the marathon's finish line. At least two people were killed and 50 injured in the blasts.

Obama has been in touch with federal law enforcement and Massachusetts officials in the aftermath of the explosions.

"Today's a holiday in Massachusetts--Patriot's Day. It's a day that celebrates the free and fiercely independent spirit that this great American city of Boston has reflected from the earliest days of our nation. And it's a day that draws the world to Boston's streets in the spirit of friendly competition," President Obama said. "Boston is a tough and resilient town; so are it's people. I'm supremely confident that Bostonians will pull together, take care of each other and move forward, as one proud city, and as they do, the American people will be with them every single step of the way."

The Secret Service reacted cautiously to the blasts, expanding the security perimeter around the White House.

He lauded the acts of the first responders in Boston, in the wake of the bombing.

4/15/2013 Related Story: Two Confirmed Dead In Explosions At Boston Marathon Finish Line