Police in the United States have shown what they believe to be the fragments of one of two incendiary devices which exploded near the finishing line of the Boston marathon on Monday.
Officers say an everyday kitchen pressure cooker was packed not only with explosives but also ball bearings and nails to make it even more lethal.
Investigators now know how, where and when the atrocity occurred, but say they still have no idea about who planted the devices or why.
An impromptu vigil has already been held. President Obama will travel to Boston on Thursday for a memorial service.
The twin blasts killed three people outright. One of them, 8-year-old Martin Richard, was waiting at the finishing line for his father to finish the marathon.
Two students were also killed. One of them was a Chinese post-graduate whose details the family want kept private. The other was 29-year-old Krystle Campbell.
176 people were injured. 17 are in a critical condition.