President Barack Obama said on Monday that the United States did not yet know who was responsible for blasts at the Boston marathon but he vowed to find those accountable and punish them.
"We don't yet have all the answers," Obama said. "We still do not know who did this or why," he added, hinting that he believed the explosions were planned but stopping short of calling them a terrorist attack.
"We will get to the bottom of this. We will find out who did this. We will find out why they did this," Obama said in a short televised statement from the White House briefing room.
"Any responsible individuals, any responsible groups, will feel the full weight of justice."
"We will find out who did this and we will hold them accountable."
The US president was swift to make on camera remarks about the blasts after being accused of being slow to respond to previous incidents, including an attempted bombing of a US jetliner over Detroit, early in his first term.
He appeared on camera just 15 minutes before main evening network television news bulletins, and after being briefed on developments in Boston by FBI director Robert Mueller and homeland security secretary Janet Napolitano.
"Boston is a tough and resilient town. So are its people," Obama said, adding that Americans would say a prayer for the city on Monday.