UPDATE: Pressure cooker bombs suspected in Boston blast

Tuesday, April 16, 2013 | 10:53 p.m. CDT; updated 7:20 a.m. CDT, Wednesday, April 17, 2013

BOSTON — Federal agents zeroed in Tuesday on how the Boston Marathon bombing was carried out — with kitchen pressure cookers packed with explosives, nails and other lethal shrapnel — but said they still didn't know who did it and why.