Which group/individual was held responsible for the bombs that arrived

Which group/individual was held responsible for the bombs that arrived in the mail at the homes of Postmaster General Albert Burleson, Attorney General A. Mitchell Palmer,

several members of the U.S. Senate, the mayor of Seattle, and federal immigration authorities and judges?
A) The American Legion
B) Victor Berger
C) Industrial Workers of the World (IWW)
D) The sender(s) were never discovered.

ANSWER
Answer: D