President Obama did not call the explosions terrorism in a brief statement to the media from the White House Monday evening, urging people instead not to jump to conclusions.
He said authorities are "still in the investigation stage."
"We will find out who did this and we will hold them accountable," he said.
Obama did not respond to questions from reporters about whether the incident was terrorism.
"We still do not know who did this and people should not jump to conclusions," he said. "We will get the bottom of this ... Any responsible groups will feel the full weight of justice."
Obama said he had ordered increased security around the United States and pledged to mobolize "the appropriate resources to investigate and respond."
His full statement is below:
Good afternoon, everybody. Earlier today I was briefed by my homeland security team on the events in Boston. We're continuing to monitor and respond to the situation as it unfolds, and I've directed the full resources of the federal government to help state and local authorities protect our people, increase security around the United States, as necessary, and investigate what happened. The American people will say a prayer for Boston tonight, and Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss.
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. I've spoken to FBI Director Mueller and Secretary of Homeland Secretary Napolitano, and they're mobilizing the appropriate resources to investigate and to respond.
I've updated leaders of Congress in both parties, and we reaffirm that on days like this there are no Republicans or Democrats; we are Americans united in concern for our fellow citizens.
I've also spoken with Governor Patrick and Mayor Menino, and made it clear that they have every single federal resource necessary to care for the victims and counsel the families. And above all, I made clear to them that all Americans stand with the people of Boston.
Boston police, firefighters, and first responders, as well as the National Guard, responded heroically and continue to do so as we speak.
It's a reminder that so many Americans serve and sacrifice on our behalf every single day, without regard to their own safety, in dangerous and difficult circumstances. And we salute all those who assisted in responding so quickly and professionally to this tragedy.
We still do not know who did this or why. And people shouldn't jump to conclusions before we have all the facts. But make no mistake, we will get to the bottom of this, and we will find out who did this. We'll find out why they did this. Any responsible individuals, any responsible groups, will fill the full weight of justice.
Today's a holiday in Massachusetts, Patriots 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 a spirit of friendly competition.
Boston is a tough and resilient town, so are its 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.
You should anticipate that as we get more information, our teams will provide you briefings. We're still in the investigation stage at this point. But I just want to reiterate, we will find out who did this, and we will hold them accountable. Thank you very much.