A contention claimed for opposing mandated taggants is that most terrorist attacks use homemade explosives (HME) which would allegedly not be tagged. Examples given included, for instance, the 1993 World Trade Center bombing, the Oklahoma City and the Omagh bombings. Contamination of the site is also cited as a problem, since different taggants might be present at a crime scene from, for example, explosives used to obtain the building materials.