UK Daily Mail 
April 16, 2013
The unnamed suspects in the tragic Boston Marathon explosion on Monday detonated two small and likely homemade bombs, which killed at least three people, including one eight-year-old boy, while injuring more than 140 others.
Initial tests showed no use of C-4 or other high-grade explosive material, suggesting that the packages detonated in the attack were crude explosive devices, federal law enforcement officials say.
The twin blasts from the bombs took place about 10 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending plumes of smoke throughout the street.
The blasts reportedly occurred near the final yards of the 26-mile and 385-yard expanse, where spectators were cheering runners as they approached the finish line.
The two detonated explosives may have been packed with ball bearings, which could explain why so many victims lost feet, calves and ankles in the horrific explosions.