Christophera uses the following link to say that Robertson, the structural engineer of the world trade center, told Newsweek the Towers had a concrete core. MSNBC - ?Painful and Horrible? this article is apparently no longer available on the Newsweek web site. Newsweek later publishes and article stating the Towers has a steel core. Robertson also is involved in this article but is not quoted saying it was a steel core just like he is not quoted as saying it was a concrete core in the earlier article. High Time - Newsweek.com the article is still on their website.