pictures, some of which appear to be taken after the scene also show the remains of a shredded black knapsack or backpack, all of which seem to be managed by authorities. they are sent out to agencies so everyone can compare notes. there is this photo as well from local affiliate wga showing a bag by a mailbox where the first bag went off at the finish line. the question tonight, was it one of the bombs? the fbi is analyzing it along with fragments of the pressure cooker. there is that and there is this. breaking news out of washington, letter to the senator testing positive both in the field and in the lab for the deadly poison lab. dana bash is going to be working her sources on that. she'll be joining us shortly. we'll also hear from the people w whose family members lost their lives. we know there was a chinese national, a grad student at boston university not far from