Being really bad doesn’t make a crime federal. The typical murder charge is a state charge. The Boston bombings were a case of Boston residents setting bombs off in Boston. So, what makes the Boston Marathon bombing a federal case?
If, like the first World Trade Center bombing, the Boston bombers were shown to be part of an international terrorism organization, executing a terrorist plot planned by this organization, that makes it an international terrorism incident and thus a federal crime.
If, like Timothy McVeigh’s bombing, this was an attack on a federal building, that would be an easy case for making it a federal crime.
In this case, according to the complaint accompanying the federal indictment, the feds are prosecuting this because bombing the marathon affected interstate commerce.
The charges are (1) “unlawfully using and conspiring to use a weapon of mass destruction … against persons and property within the United States used in interstate and foreign commerce and in an activity that affects interstate and foreign commerce, which offense and its results affected interstate and foreign commerce. …”
The other charge wa causing injury and death while using an explosive to destroy property “used in interstate and foreign commerce and in activity affecting interstate and foreign commerce.”
“The explosions had a substantial impact on interstate and foreign commerce,” the indictment argues, because the marathon attracts runners and spectators from all over the world.