Two bombs ripped through the crowd at the finish line of the Boston Marathon on Monday, killing three people, maiming others and injuring more than 100 in what a White House official said would be treated as an "act of terror," Reuters said.
It was the worst bombing on U.S. soil since security was tightened after the attacks of September 11, 2001, and President Barack Obama promised to hunt down whoever was responsible for the attack on a day when tens of thousands of spectators packed the streets to watch the world-famous race.
No one has been arrested and the White House official said it would have to be determined whether the attack came from a foreign or domestic source.
Investigators found what could be five additional, undetonated explosive devices around the Boston area, the Wall Street Journal reported, citing two unnamed people briefed on the investigation. The evidence had yet to be fully analyzed, the Journal said.
Many runners were heading for the finish when a fireball and smoke rose from behind cheering spectators and a row of flags representing the countries of participants, video from the scene showed.
Some suffered shrapnel wounds and amputations and will require repeat operations in the coming days, said Peter Fagenholtz, a trauma surgeon at Massachusetts General Hospital.
