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The current building opened in May 2006. An elevated walkway connected the building to the World Trade Center plaza. When the building opened in 1987, Silverstein had difficulties attracting tenants. 1988, and became the main tenants of the building. The debris also ignited fires, which continued to burn throughout the afternoon on lower floors of the building. The collapse began when a critical internal column buckled and triggered structural failure throughout, which was first visible from the exterior with the crumbling of a rooftop penthouse structure at 5:20:33 pm.