The morning of April 19, 1995, marked the worst terrorist attack ever experienced in the United States, when a truck bomb destroyed the Edward P. Murrah Federal Building in Oklahoma City, killing 168 men, women, and children. Five years later, to the day, a memorial at the site of the tragedy was dedicated to the memory of those who lost their lives, to their survivors, and to the rescuers who came to the victims' aid.

The moment of that explosion has been framed for posterity in concrete. Entry to the memorial is through the Gates of Time, two pairs of 56-foot-long walls up to 47 1/2 feet high that define the moment of the explosion. The buff-colored concrete walls are 22 1/2 inches thick at the base and taper to 12 inches at the top. The walls of each pair are 14 feet apart and contain entry portals 7 feet wide. Forms for the gates were made of 3/4-inch-thick high-density-overlaid plywood supported on a framework of patented steel beams and studs.