From previous reports we know the cooker was packed with nails and ball bearings, the metal trash can where the bomb may have been hidden probably added to the shrapnel.

Via CBS News:

Two bombs blew up seconds apart Monday at the finish line of one of the world’s most storied races, tearing off limbs and leaving the streets spattered with blood and strewn with broken glass. Three people were killed, including an 8-year-old boy, and more than 170 were wounded.

A law enforcement source told CBS News that one of the explosive devices appears to have been placed in a metal pressure cooker (a metal kitchen pot with a locked down top) which had been placed in a black nylon bag or backpack. Investigators also found pieces of an electronic circuit board possibly indicating a timer was used in the detonation of the bomb.

A law enforcement official told CBS News that the two bombs that exploded were made to look like discarded property. It is still unknown if one or both bombs were in garbage cans. One may have been on the sidewalk.