From paragraph 3 of the complaint:

(1) unlawfully using and conspiring to use a weapon of mass destruction (namely, an improvised explosive device) 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 (including, but not limited to, the Boston Marathon, private businesses in Eastern Massachusetts, and the City of Boston itself), resulting in death, in violation of 18 U.S.C. 2332a;

(2) maliciously damaging and destroying, by means of an explosive, real and personal property used in interstate and foreign commerce and in an activity affecting interstate and foreign commerce, resulting in personal injury and death, in violation of 18 U.S.C. 844j.

See here for my previous thoughts on the commerce-clause hook in this case.

Update: Notably, the entirety of the complaint is based on surveillance imagery–much of it crowdsourced–and introduces nothing from any Miranda-less interrogations conducted.