However, it is an assumption that Leahy and Daschle were first and (so far) the only targets. They could merely be the only two senators who chose to publicize their letters.
The Senate may have agreed to abide by a voluntary gag rule for investigative purposes. Some targeted recipients may not even have been told about "their" letters discovered in the quarantine drums. And letters may be yet discovered--all with the same postmark date.
We don't know that this Leahy letter was just now found. He could have been the 20th senator to have an envelope addressed to him, but only the second to yap off about it.
An American would go for the kill, not the terror factor. Why would an American want to terrorize his own countrymen? Why would an American waste his first anthrax "bullet" on the National Enquirer?
These terrorist attacks are a right winger's worst nightmare come true. It's only led to more government intrusion and the establishment of forces that will one day most likely turn against the militia groups. This was evident after the OKC bombing and the ensuing legislation.
The right wing has nothing to gain from this. And, if they were going to go to all this trouble to shoot themselves and the Constitution in the foot, I would think they wouldn't fool around. They would go after their worst enemies first with no warnings.
Whoever is doing this wants to destroy America, not destroy the destroyers of America.