I found the answer in the Wikipedia article[1]: > For the next three months, a construction crew welded two-inch-thick steel plates over each of the skyscraper

I found the answer in the Wikipedia article[1]: > For the next three months, a construction crew welded two-inch-thick steel plates over each of the skyscraper's 200 bolted joints during the night, after each work day, almost unknown to the general public. [1] http://en.wikipedia.org/wiki/Citigroup_Center http://en.wikipedia.org/wiki/Citigroup_Center