Barack Obama has responded to a series of crude bombs which rocked the finish line of the Boston Marathon on Monday evening.
The US President was grave as he addressed the nation from the White House press office. "We will find out who did this and we will hold them accountable," he said.
"Make no mistake we will get to the bottom of this and we will find out who did this and find out why they did this.
"Any responsible individuals, any responsible groups will feel the full weight of justice".
Mr Obama, who carved out his law career in the state of Illinois, paid tribute to the strength of the Boston community.
"I'm supremely confident that Bostonians will pull together, take care of each other and move forward as one proud city," he said.
The president was informed of the incident 10 minutes after the first explosion. He called the attacks a "tragedy" and a "senseless act."
Boston descended into chaos on Monday evening after two bombs exploded at the finish line of the city's marathon. Runners and well-wishers were seriously injured.
According to reports, three people died and at least 140 were hurt in the blast, which still hasn't been attributed to any group.
British police have now stepped up security around Sunday's London Marathon which is the next major international marathon to take place.
Twitter reaction to Boston tragedy
- Mark Wahlberg: Thoughts and prayers with my hometown Boston today."
- Barack Obama: "On days like this there are no Republicans or Democrats - we are Americans, united in concern for our fellow citizens."
- Katherine Heigl: "No words can properly express my sorrow and rage over what happened in Boston. I pray there is Grace in these ugliest of times for all those injured and suffering. My love and support is with you."
- Sheryl Crow: "I am just sickened to hear about the news in Boston. My heart and prayers go out to everyone affected by this tragedy - Sheryl"
- The Saturdays: "Such heartbreaking news from Boston. Sending our love to everyone affected."
- Alicia Keys: "Shaken…My thoughts and prayers are with the city of Boston and those affected by this tragedy."
- Justin Timberlake: "My thoughts and prayers are with everyone in the city of Boston and all of those affected by this terrible incident."