But 9/11 wasn't the original catalyst. On April 19, 1995, Timothy McVeigh detonated a truck bomb in front of the Alfred P. Murrah Federal Building in Oklahoma City, killing 168 people. President Bill Clinton responded by directing the Justice Department to produce a report on the vulnerability of federal facilities to acts of violence. The so-called "Marshals Report" recommendations led to new construction standards and a paradigm shift in building design.
Above, the north side of the Murrah Building after the truck-bomb explosion.
The bollards blocking the 1600 block of Pennsylvania Avenue from traffic can be lowered into the ground, providing a subtler and more flexible form of protection than concrete barriers. As architecture critic Witold Rybczynski writes, Washington has recently become victim to "bollard envy ... where the degree of protection becomes a symbol of bureaucratic status."
Tea Party members gather on the grounds of the Washington Monument on April 15, 2010, to launch a People's Tax Revolt. They're sitting on the monument's curving stone retaining walls, designed to provide unobtrusive perimeter security.
Washington's increased security barriers aren't always foolproof. On May 4, 2006, Rep. Patrick Kennedy, driving drunk, hit a security barrier in front of the Cannon House Office Building.
Police officers stand in front of the U.S. Supreme Court and its iconic marble steps on Dec. 1, 2003. In 2010, the Supreme Court decreed that the public would no longer be able to enter the building through its front entrance -- visitors must use a special security entrance through a side door instead.
