explosion. >> that explosion killed 14 people. 10 of whom were first responders. nearly 200 hurt. and dozen of homes are now gone. today assistant fire marshall said investigators may not have concrete answers any time soon. >> the investigation will take so long because it's done on a scientific method through a scientific process. it's very methodical. one way to compare this archeological dig. we are layering through everything we are going to try to get the answer. >> shepard: president obama will attend a memorial service in the town of west on thursday. fbi agent says investigators did not find ricin inside the home of a mississippi man. the one accused of sending poison letters to president obama and u.s. senator. he also says investigators did not find ingredients for making poison or computer searches on how to make ricin. this on a hearing for the suspect paul kevin curtis.