Speaking to motive for the Daschle and Leahy letters, there might be multiple motives.
The letters likely influenced their decisions on upcoming legislation like the Patriotic Act, and both had considerable power to promote or suppress legislation. The letters were likely intended to frighten all Congressional Members, whose office buildings were affected, and who, like the rest of us, didn't know what would come next.
Senate Majority Leader Daschle also may have been targeted because of his position regarding the Defense Department's mandatory anthrax vaccine program. In a letter he wrote to Defense Secretary Rumsfeld dated June 21, 2001, Daschle sought to weigh in on an ongoing DoD review of the vaccine program. He criticized the vaccine over its safety and efficacy, as well as persistent problems with its manufacture.
Did someone who read the letter, or was otherwise aware of Daschle's concern about the vaccine, decide that an anthrax letter might shift his thinking on the vaccine -- as well as the Patriot Act and other legislation?