The FBI released images of one of the devices that exploded at the Boston Marathon.
Investigators say the pictures show the remains of a pressure cooker that contained a bomb.
One photo shows a shredded black backpack and what appear to be metal pellets or ball bearings.
A Boston law enforcement source said two identical pressure-cooker bombs -- each with the capacity to hold six liters of liquid -- were set off.
The FBI said they contained BB-like pellets and nails, causing even more damage.