The bombs at yesterday's Boston Marathon were made from pressure cookers packed with metal shards, nails, and ball bearings, then placed in duffel bags and left on the ground, a person who has been briefed on the investigation tells the AP. NBC News has basically the same information, citing law enforcement officials; it adds that the bombs were set off using timers. The AP's source says officials don't know what was used to actually set off the explosives, which were low-grade (their shrapnel was moving at 3,300 feet per second or slower, NBC explains). Similar pressure-cooker bombs have been used in Afghanistan, India, Nepal, and Pakistan, as well as in the attempted Times Square bombing in 2010.
At a press conference today, the Massachusetts General Hospital trauma chief confirmed that the metal fragments pulled from victims were likely pellets or nails specifically placed in the bombs to inflict harm, the Boston Globe reports. "I think it’s unlikely they would be so consistent if they were pulled out from the environment," he explains. Some victims were hit with as many as 30 or more pieces. An emergency physician who worked on some of the victims also described seeing small ball bearings, which he said "clearly were designed to be projectiles that were built into the device." The timing was also likely chosen to maximize injuries, as the marathon's four-hour mark is typically crowded with recreational runners crossing the finish line as their friends and relatives cheer nearby. (Officials earlier said there were no unexploded bombs found, contrary to early reports.)