This, from Time Magazine , here in the U.S.: "...It turns out that federal law defines “weapon of mass destruction” in extremely broad terms. The relevant statutes define almost any significant explosive device as a WMD. That specifically includes bombs, grenades, mines, and small rockets and missiles. The pressure-cooker bombs planted at the Boston marathon and the explosives hurled at police on Thursday night would almost certainly qualify.
Needless to say, the law also covers weapons more commonly considered to be WMD, including “any weapon involving a biological agent, toxin, or vector” and any weapon “designed to release radiation or radioactivity at a level dangerous to human life.” (More here on what qualifies as an agent, toxin or vector.) But there is no indication that the Tsarnaevs had access to such materials..."
Additionally: "...The federal government has executed just three people since 2001. Among them was Oklahoma City bomber Timothy McVeigh, who was also charged with use of a weapon of mass destruction and who was put to death by lethal injection on June 11, 2001, at a federal prison in Terre Haute, Ind..."