Boston - Hundreds of law enforcement officers are combing through evidence, looking for anything that would point to a possible suspect or motive behind Monday's bombings at the Boston Marathon.
The two bombs left three people dead and more than 170 injured.
The first explosion happened Monday afternoon around 2:50 Eastern time, just as thousands of runners were finishing the race.
About 12 seconds later, a second explosion occurred.
Officials say the blasts were about 50 to 100 yards apart.
A federal law enforcement officer says both bombs were small but had a powerful impact.
People were rushed to emergency personnel, windows were shattered and dense plumes of smoke rose over the street.
Authorities are continuing to sweep the entire length of the marathon for any evidence.
They say there are no known additional threats at this time.